5+ years of left-sided chest, back, and neck discomfort

Hi everyone, for about 5 years now I’ve had this strange discomfort in my left chest that sometimes radiates up into my back and neck. It’s really hard to describe, but it feels like a mix of a dull ache and something moving, shifting, or even beating inside that specific area. I’ve also noticed that my left side is actually getting smaller than my right side, my left arm posture looks different, and in my lower chest it feels like there’s literally a gap in the muscle. What’s weird is when I’m moving around, doing things, or running, I don’t feel anything at all. But as soon as I sit down, that discomfort in my chest, back, and neck comes right back. If I press down on those areas the feeling goes away, and when I lie down on my back to sleep I always have to put a pillow or my hands on my chest to even be able to fall asleep. I really don't know what this could be, so has anyone experienced anything like this or knows what might be going on?