Practicing should never hurt, neither physically nor mentally. I hope this sounds obvious to you, but to me it did not.
I've just realised in the world of classical music, we never talk about health in an appropriate way. When I got pain in my hands due to playing the reaction of my teacher was 'this happens to all of us' and interpreted it as a sign that I practice long. When I almost couldn't practice anymore because my mind didn't allow me to (I had so many negative thoughts I couldn't start playing) the reaction was 'well, we all have that sometimes, it will go away'. Such situations and reactions shouldn't be normal. When your body hurts, this is a sign that something is wrong. You should take that seriously or risk serious damage which eventually leads to that you can't play anymore at all. When your mind doesn't allows you to start playing you should speak to someone. There is probably a reason for that.
Don't get me wrong, my teacher is a very kind person, he just didn't know better.
To be honest I hope this post is unnecessary because you all know that. But I know so many musicians, teachers and children who don't that I have the urge to share that.
Seconding! Please look up how to stretch your back and your hands!
Yes, don't play through pain and I would add don't even play through significant discomfort, especially if you're advanced enough/practicing enough that you know what feels right to you. I made this grave mistake and played in extreme discomfort for almost a year and then got a severe nerve injury from it. I never pressed my teacher to change how I was playing because I wasn't technically in pain for most of the time, so I wasn't taken seriously.
Musician health is one of the most important underrated topics in music. It's so so important and a professional with an injury faves similar problems as a sports professional, only that I'm sorta there is much more awareness around it. I'm in my 40s now and play the viola and I'm glad I came across this topic around the time I switched from violin to viola (if the violin isn't ergonomic, the viola is much less so and less forgiving of bad technique). Being careful and taking care of your body is very much worth it.
I second this! I remember when I was helping someone learn how to play their horn they played until their lips bled. I told them to not play and the teacher practically told them to keep going. Never play so much that it physically hurts to that extent. You will do more damage then any real progress and you risk never being able to play again.