I think you have every right to be upset because it's annoying. But I think you are getting a bit too annoyed. For your own mental well being, try to let it go. Give people the benefit of the doubt, and if it really bums you out, stop hanging out with them. Accountability for being flakey is accepting that you will lose friends. So stop being their friend.

Also, learning to navigate this world with a mental illness is a learning curve. I know I kept flaking on my friends and when I did hang out with them I'd carry my problems with me. And of course I lost tons of friends. I learned how to value people around me and ask for help as well as give help.