I appreciate your email and believe this is an issue that you and your boyfriend must face together.
The nature of your relationship with your boyfriend is such that his mom feels compelled to criticize. I believe this is an opportunity for you and your boyfriend to join forces and talk to his mom directly. Actually, the best place for this is with a third party present, like a couples or family counselor, but the two of you alone would be okay. You being hassled by his mom is something your boyfriend needs to help you with.
If you try to do this on your own without help from your boyfriend, I think it will continue to generate problems. I would talk to your boyfriend and plan to have a sit-down meeting with his mom about your concerns.