outside the game:
most of the time Im having an off day its definitely because I didnt sleep well, am not hydrated, and just simply didnt stick to the routine i've setup for myself. however, if youre adhering and the aspects outside the game are on point but youre still having an off night, then its time to talk about mentally resetting yourself in game.
in game
if you have a bad start and you are able to mentally recollect yourself in the midst of a game, you'll be able to bounce back. dont think too much on whats already happened but rather use that ask urself what needs to change. there have been plenty of times where i started off mega negative and then completely turn it around to wipe or roll a team.
and remember to not over-practice or under-practice your DM, i feel like its really easy to go into games either super cold or super burnt and depending on how much you play it can either be for better or for worse. if youre struggling to understand what to do in some situations as well and just cant come to a resolve its time to bust out those demo reviews. lastly, also ask if your teammates can notice something not right with your gameplay, this can help alot with different perspectives into play.
but with that being said good luck