I appreciate you sending this important and yet complex question to us.
I think it is time for you to assess what it is you want in your life and where this relationship is going. The foundation of your relationship was begun by lies, betrayal, and infidelity. These are very difficult traits to overcome and forgive.
While I admire your boyfriend’s eventual honesty, and the fact that you say you’ve forgiven him, the underlying difficulty is the fact that every time there is a problem all of the old hurts will surface. This makes it difficult in particular because whenever his old girlfriend is not in a good place she can start the pot.
I would do two things: First, I would ask yourself if this whole situation is indeed what you want to be coping with — and if it is what you would like for your future within the relationship. I don’t know that these questions have been fully addressed. Second, I highly recommend couples counseling because of the complexity and ongoing nature of your concerns. Each developmental milestone and every unresolved conflict with his girlfriend has the potential of deeply activating the issue. The two of you are going to need excellent communication, which can be fostered in couples counseling.