My Mom is my number one trigger. When I was in high school I ran away from home all the time because of it. Finally a minister at my church tried to help me, but I knew better than to tell the truth. I didn't tell anyone about my homelife until I was 18. That fact of the matter is that a therapist has to report any abuse of a minor. But on the other hand, your therapist has to keep what you say in sessions confidential unless they feel that your life is in danger. I would let the therapist know upfront that you do not want your problems with your Mom to be discussed outside of your sessions and without your permission. I would try building a relationship with the therapist until you become more comfortable in sharing your relationship with your Mom.
In dealing with your Mom, maybe you could write a letter explaining how she triggers you. That way you wouldn't have to worry about getting into an argument with her over it and you would be letting her know how she can help you. There are no easy answers to this. I am 35 and it still takes me weeks to recover from a visit from Mom. That sounds so awful, but it's the truth. I have recently started standing up for myself with my Mom and she has backed down some.
If you can, approach her and let her know how her behavior is affecting you. Is your Mom educated on Bipolar Disorder?