Is there a way to fix this shoulder button? I dont know what happened, but all of a sudden sometimes the button stays stuck, and I pull on it a little or push down on it hard and it works for a while, but then it stops working again



You could try watching this vid. And disassembling it yourself to see as to why it's sticking. It could be buildup from your fingers or just something broke? Only way to tell is to take a look I suppose.
this will void warranty I would imagine
https://www.ifixit.com/Teardown/Nintendo 3DS XL 2015 Teardown/36346