https://www.youtube.com/watch?v=V5DA3RM5WAM&index=5&list=PLozaDKp8It3zDHjWDdDR_EpGcNFNcLBbQ
TL;DR - find ways to raise the skill/consistency/whatever bar of your worst play. You can do that through repeated experiences so you get used to playing at your worst, flowchart your play so you can fall back onto something you're comfortable with, find ways to focus and get out of playing your worst, etc. You'll probably have to do all three+others to get past it.
But you should also realize that blaming your losses on "worse play" can be bad.
click. You can acknowledge that you weren't playing your
best, but a loss is a loss, and it's better to move forward and accept that as your
base level. That is...stop thinking about your
average level from friendlies, because however true that is to your actual average - it's easy to be biased and think better/worse of ourselves - that's irrelevant if you can fall way below that for _____ reason in an important set. Be mad that your base is bad so you can't use "playing below my average" as an excuse, and then move from there.