I never have boxed but I never liked wearing a mouthpiece in football. Not like I gagged regularly at all but I did every once in a while and I was never really nervous playing football, so some people might just be annoyed by mouthpieces. Although not saying your nerves don't play a part.

Thing is its also probably in your head, happens once or twice and where you wouldn't think about it before now you're like waiting for it to happen. Getting a new mouthpiece might be helpful for that alone.

This guy on the ultimate fighter this year had a problem with it and went to a hypnotist. I don't think it worked, haha.