Dawr,
I think Sky is right. If you are of age, then you don't need your mother's approval to go to any doctor or therapist you want provided it's covered by the plan.
I also think Sunny has a point with respect to having the MD call your mom, but that will only work if she is open to it.
Sometiems, parents don't know what to do because their experience has been so very different. Your mom was probably brought up to not ask for help. I think you have done an amazing job so far, standing up for yourself and taking care of your needs.
The next step would be to find out if therapy is covered by the insurance and how to make the co-pay.
Keep posting.
Peace