Since you’ve played puff for so long, you have developed muscle memory to press UpB when you’re in a concentrated moment in a match because of your habits. You should opt to use other moves rather than resorting to Sing to put your opponents to sleep. The only time you should ever use sleep is if you can land a hard read on your opponents, which gives you a free opportunity to land a rest. I can understand the portion where you claim that in a heated situation that it’s difficult for you to calm down, and everyone is like this including professional players as well. One way is to not force yourself to play aggressively against the opponent, as Jigglypuff isn’t a character that you can easily play aggressively. Play it slow and try your best to figure out your opponents game plan, and figure out a way to find an opening. Blindly rushing against the opponent will more likely cause you to UpB instinctively as they rack up easy damage against you.