Shrugging and hunching back.
I've started to develop a bad habit of raising my shoulders and hunching my back when I play passages that are fast or are difficult for me. My teacher pointed it out a while ago and I've been trying to correct myself while I practice, but the issue is I don't notice I am doing it in the moment. It isn't until I finish a passage or two that I realize I'm hunching over. I feel like this will turn into something completely damning if I don't stop it now, but trying to spot it and correct it when it happens isn't working so far.
I sometimes practice while standing on a balancing board. It is almost impossible to hunch when standing on the board.
How's your vision? Are you leaning into the music stand because you can't see the notes clearly? A prescription for glasses or lenses that let you stand back from the music and still see clearly might help keep your posture upright and open.
Are you trying to grip the violin with your shoulder? Perhaps a taller chin rest or different shoulder pad might reduce the temptation.
I’ve tried taller chin rests and shoulder rests and different styles as well, and none of them felt as good as my current set up. I do have poor vision, but this happens whether I’m sight reading or not. If I’m completely honest, it mostly happens when I use vibrato on difficult passages, so I think it’s causing me to tense up. But I don’t tense up when using vibrato on simple things.
It is very important to play the violin, both sitting and standing, with a straight back.