I agree that she has boundary problems. Do you live with her? Do you have a therapist - this is the type of thing that is PERFECT to practice with a therapist, in my opinion.

What I think you need to do is sort of what you'd do with a little kid.

She starts complaining about your boyfriend. You say (firmly!): "Mom, I understand that you don't like Gerald. You've made your position very clear. I like him very much, and am not interested in hearing any more negative comments about him. If you can't stop criticizing him, I'm not going to spend time with you."

If she then continues, you WALK AWAY. You can say something like, "Mom, I told you that I wasn't interested in hearing any more criticism of Gerald. I'm leaving now."

The key is to completely disengage with her, so she gets no more attention or reinforcement from her complaining, and so that you get away, and don't get sucked into an argument.

You don't have to repeat step 1 each time, but every time you're together and she starts criticizing your boyfriend, you do step 2 again - tell her that you are not interested in hearing it, and physically leave.

I'm not a therapist (but I read alot!) - so I can't make any guarantees. But it seems like this might get the idea through to her that her behavior is not acceptable, and that you won't tolerate it.

BTW, I'm glad you've found somebody that you like, and are in a good, loving relationship, you deserve that! I'm really sorry that your mom can't be more supportive, or at least keep her criticism to herself. I almost wonder, given she has no friends and your dad is gone, if she's afraid that she's losing you too now? I don't know if that would change your approach to her at all, it seems like she's so inflexible and unwilling to look at her own stuff.. it really does make it hard.

Good luck!