[¯|_(ツ)_|¯]

I know that she may not like the idea of you going to solo-sessions, but in the end, if you think that will help you get the most out of your sessions, then it's best to be honest. You can say "I think it will help to talk to -psychiatrist name- by myself for a while," or if you use the idea of slipping your psychiatrist a note, you can keep it simple and say "I want to see you privately."

I think it may be worth discussing things with your mom as well, separately. Let her know you appreciate that she's worried and only wants to look out for you, but right now you don't feel as if you are getting better. Let her know what WILL help you, and she may be more likely to listen?