you probably get tired
Why do I play either really well or completely disastrously?

There are many factors that can go into that like mental fatigue/tiredness, your opponent could be playing well, maybe emotionally your not there, or maybe it's a learning curve related thing. Just don't worry performance can be inconsistent just try your best, good luck!
i play with alcohol i dont think mental fatigue/tiredness matters so much,when i play bad i usually play out of my comfort zone.the "comfort zone" is a set of movements that u have seen million times and u can answer fast and effective. when the moves change its when problem starts since u have to think, spend time and maybe make a wrong move.also how happy or sad u are really affects chess
After 2 good matches I will play a game where I hang my queen or rook because I created a situation where I can’t escape