I once made the mistake of getting one two sizes too small (this was after, unbeknownst to me, Underworks completely changed the cut and size of their binders a few years back)... I never wear it. I can't. I don't even think I could, were I to shed ALL of my weight, because it pinches the living hell out of my shoulders.
You need to stop using that binder immediately, and get one the correct size. The only time you can go ONE size smaller, is if you're in-between the two. Binding too tight, and for too long, can actually damage your ORGANS never mind pulling/tearing muscles. You can even crack ribs. It's just an all around BAD idea.
If you really looked around, NO man has a perfectly flattened chest. That is not the object of binding. The correct size is enough. I'm unfortunate enough to have to deal with D's, and yet I never have a problem being 'misgenered' with a correctly sized binder. They just look like sculpted pectorals - the goal. And I can wear mine clear until bed (I can and even have worn it while working out too) with no discomfort or pain whatsoever (but I NEVER sleep in mine). So, again, the correct size IS enough (even for D's!). Get a correctly sized binder, bottom line.