A good worry. I don't actually have any knowledge on this, but I would assume that in family therapy, the emphasis would not be on processing the past together overly much--I would guess that the goal would be to find out why your relationship has this stuck point and find a way to refocus it on the present. Sure, you'll address the past, but I don't think it would be processing in the same way as occurs in individual therapy.

You're right--talk about your thoughts with your individual counselor. I can't stress how great it is that you are wanting to take this step with your mom. I hope she's willing to go with you and help your relationship grow. I wish I had the courage to approach family therapy with my mom.