I want to tell my mom I need therapy
What is your reason for wanting to tell your mom that you need therapy?
Because she is part of the root cause of my problem
If you're in a toxic situation you can't get out of, then therapy might help you get the comfort you need and come to a mutual understanding with your mom. Communication is the key to a successful relationship, so sit down and talk to your mom about this so that they understand your view.
It's not that serious. I don't think talking to her will solve the problem.
You do not know unless you try it. Besides, if you go to therapy your therapist will without a doubt challenge you to talk to your mother about your situation.