If you get nervous and jittery in MGE you will get nervous and jittery other places as well. What you need to do is play through it and focus on relaxing if you can. The only real fix for this is experience, over time it will get better and your focus will improve.
If you ever plan on going to a competitive LAN you will feel the same there, so this type of practice can help you stay relaxed. This is one of then biggest reasons why people talk about having "LAN experience" as well, because you can get used to it once you know what to expect.
TLDR; practice through it, do your best, stay positive!
P.S. the point of MGE is to lose; once you find a person who can beat you you should play them as much as possible, struggle to work out why they can beat you and think of ways you can improve so you can beat them.
P.S.S. Don't be a baby, play on your main account under your actual name and don't give a shit who beats you.