Hmmm .... stress and poor posture generally, along with over gripping the instrument are the causes of pain. Sit up or stand up straight, keep your wrist arched not flattened, experiment with how little pressure it takes to clearly note your instrument. I do not know where your instrument came from but a visit to a violin shop may add to its "playability". Practice in front of a mirror to check posture "issues". Play with loose fingers loose wrist loose shoulder and elbow. Play some scales with your bowing elbow trapped on a door frame to experience what may be done with wrist and fingers. Relax, breath, take your time .... fiddling is a lifetime experience.

Make sure your left elbow is tucked well enough under the violin’s body. If it sticks out like a chicken wing, it puts more strain on the left hand because you have to extend the ligaments more to play fingered notes. Possibly the most important aspect of left hand technique is economizing motion, so keeping the fingers in a position that allows them to simply drop straight down onto the fingerboard is critical.

Also, make sure that the neck is held in the proper spot on the hand between the base of the first finger and the upper half of the thumb. There should be a gap under the neck to the web of skin between the thumb and palm. Dropping the neck down too low causes the fingers to curve excessively to reach the fingerboard, and that strains the muscles.

Keep the hand perpendicular to the neck, not slanted back toward the scroll. The thumb is a good indicator—if it points the wrong way, change your hand position so that it’s straight. It’s possible to play with bad posture, especially because fiddle tunes require less technical command of the fingerboard, but bad posture risks injury and it prevents the development of technique if you venture into different repertoire.

yep, and…
Do not squeeze the neck - I found that my bad habit is battening down with my thumb and “hanging on” while playing fast. Be aware what your thumb is up to…it should not be pressing tensely against the neck!

As Richard said - What is the lightest finger pressure you can use, and still produce a nice clear note? It’s good to find that point…you may need a lot less energy/force to play, than you think.

Holding fiddle/violin out to side or body (roughly 45 degrees) is less tension for your left wrist/elbow than holding it out in front of you…holding it out more in front of you requires more of a “twist”…

There are lots of stretches online…One of my favorites is loosely curling my left hand fingers into a fist, bending the hand down and and holding it out to the side/behind my back after playing. There are many more stretches and warmups online if you search “stretches for violinists” in YouTube.

( Since you have had stress with mandolin and dobro too … are you playing crazy amounts? Maybe it needs to be a limit of 20-30 minutes a day for now? Also - There are charts for return to playing after injury. It starts with 5 minutes once a day (like, slow scales) and builds up to 10 minutes, then to 15 minutes twice a day, etc). This chart was really helpful to me after I fell off my bike and landed on my left hand/wrist! )

Lastly you may have a trigger point or muscle knit in your arm there? Try feeling around and if there’s a sore hard lump around there, give it some warmth and massage…

That you've also had problems with mandolin and dobro suggests that the trouble is not related to a particuar instrument or technique, but more with your general approach to playing. I don't know how to tell you how to achieve it, but relaxation is key to playing music. It always strikes me that the best players make it look easy, even if their music is quite intense.

Forearm/hand strengthening...!!! Get one of those cheap, squeezable thingys.. and build up some left hand muscle... right hand also while you are at it.. It will help a lot...

Some players find holding the fiddle neck in the palm of the hand helps.

shift and use stronger fingers. And when you feel like you ought to be emphasizing a note, make that note sweet and clear rather than scrunching down on it.

I've found that using a good shoulder rest/chin rest combo and adjusting them to my personal liking goes a heck of a long way toward keeping my left hand, arm & shoulder relaxed and happy. It allows my left hand to support the fiddle on the index finger's knuckle with close to zero stretching or twisting weirdness. It allows my bowing hand & arm to find their sweet spots, as well.

Exercises, stretches, treatment? Maybe, but... do those address the cause of actual issue? Prevention.

Rather than power way through, no pain no gain idea. Listen to body. Pain, ache, discomfort is your bodies way of telling you somethings off, and to reexamine what needs to happen.  It really shouldn't require much in left hand muscle strength, nor contortion, outside comfortable range of motion and flexibility.

I suspect it's probably idea/need of "holding" the fiddle; like squeezing with thumb; locking the wrist; and/or chin/shoulder (latter which can cause slight pinching in neck/back, that migrates down to elbow and fingers). As well, "hold" might be a sense to need locked in to fixed ("proper?") position, maybe neck glued to hand; thus lack flexibility, freedom of movement.

It can also be idea of how much down finger pressure/strength is needed in stopping the note, which then needs more counter pressure. As other point out seeking a minimal light touch helps.

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Alternative can maybe think in terms of more resting, supporting and balancing the fiddle; just preventing slippage.

To me, discomfort and pain are two different things. When you’re new to playing the violin, especially as an adult learner, holding the instrument will not be particularly comfortable in almost any hold because it’s just not an instrument designed for player comfort to begin with. Playing posture is all about getting the best sound out of the instrument in the most efficient way, drawing on the wisdom of great players and teachers over centuries. The indirect result of this is that it takes a while to become comfortable with the instrument—instead of moving the instrument around until it feels comfortable to you, you conform to it and gain comfort as your muscles and ligaments develop. Once you’ve learned to play, the discomfort disappears. This is how it’s possible for people to spend 10+ hours a day practicing (not that that’s necessarily a goal to which the average player ought to aspire, but it shows what’s attainable).

Pain is to me an alarm from the body that it is in danger, and when it shows up in violin playing, it’s typically caused by bad form, exhaustion, or repetitive strain injury (arguably also caused by bad or exaggerated form).

One of my violin heroes developed right shoulder issues at the end of his career; this was a result of his extremely high bow arm when playing. It looked amazing on screen to play that way and it added showmanship to his performance. It eventually made it difficult for him to raise his arm much at all, however this only came in his 80s, and he spent 10 hours a day practicing for decades, even while he was producing and starring in his own TV show and concertizing extensively. Because his technique was so brilliant, he was still able to perform with the shoulder issues, and his final performance was masterful even though he was playing through pain.

I don’t recommend playing through pain in general, and I think it’s good to stop and take a break if that’s happening. But I think discomfort should be treated differently. Learning to play is a process, and it involves a transformation of the body over time. Just as the body changes to accommodate a wider range of motion as you do stretches over a period of time, it adapts to violin posture. The first time you do a new exercise, it’s likely to feel awkward and uncomfortable and may even put you at the threshold of pain. It’s crucial to be able to stop when the threshold is crossed, and knowledge of that limit only comes from approaching it. As the body changes, the limit moves. One of the frequently forgotten benefits of practice is familiarizing oneself with these bounds.

I often come across recommendations to “do what feels natural” or “do whatever works for you,” and I think those are misleading and dangerous suggestions. Since the instrument is not particularly ergonomic, there isn’t really a “natural” hold for it, so doing whatever seems comfortable is really a matter of trying to find what’s easiest at the expense of what actually works. The problem with the idea that one should determine one’s personal posture presupposes that one is able to make such a determination, which is a ludicrous idea and puts an unreasonable burden on a beginner. Another major problem is that doing what feels comfortable can be a recipe for disaster in other areas; bad form with one hand may lead to issues with the wrist, arm, shoulder, neck, or the other hand.

We musicians spend lots of time practicing and playing our instruments, so we are prime candidates for repetitive motion injuries. Athletes put considerable effort into training to prevent injuries, and they make sure to warm up and stretch before their games. We musicians don’t do much of that, we’d rather just play.

Janet Horvath wrote a very good book about injuries that musicians suffer. It’s called “Playing Less Hurt”. When I first started playing fiddle several years ago I had some of the usual ergonomic problems, and this book helped me understand what was happening and how to prevent problems. Now I play with no pain or discomfort -- if I hadn’t read that book I probably would have quit playing fiddle.