In high school I just kinda started doing bad at things that I used to do well but on purpose. I was a band nerd and had been first or second chair for a very long time and one day I decided that I was going to bomb the chair placement tryouts and let somebody else play the hard parts of the music (it was a public high school.. nothing that we played was very hard).
I probably could have just asked to play third part but it was kinda ... necessary to go through the process of failing. Even though I did it on purpose, it still felt very bad. Which was a weird thing to feel myself going through.
Maybe find something hard that you can do on your own that you know you're going to fail at. Some stupidly hard video game (no cheats or walkthroughs) or read a very difficult book to comprehend all the way to the end without stopping to look anything up or read somebody else's synopsis. Just constantly tell yourself that its okay to not be good at a thing.
Fuck... if your self description is anything to go off of, if you really want to dive blind off of a cliff, go find a karaoke bar. Walk in the door, either ask somebody to pick a song for you that they won't mind you mangleing or pick one at random. The only rules are that you have to keep trying to sing the song, no matter how bad you fuck it up and you stay on the stage until the song is over.