I'm really bad at knowing when clothes fit. Even a t-shirt, I couldn't tell whether I was a small or medium until I went into a fitting room with multiple mirrors which showed me how ridiculous I looked in medium. Now I'm having this problem with a long coat I bought (only medium left in stock), can't tell whether it fits baggy or it doesn't fit. I also have trouble with skinny jeans and shirts. Is there any guide for how clothes are supposed to fit?
Take a pic of yourself, and contemplate posting it in some forum's WAYWT (maybe here, maybe Reddit, maybe Sufu, maybe Dressed.so). You will critique your look much harder lest others do the same to you, and you'll be doing so from a different perspective than you usually view yourself each day.