I have to agree with Taylala; your mother is the one being ridiculous and honestly quite childish here, and I'm afraid there isn't much you can do other than sit down with her and have a calm and mature talk with her about why you care for this person and why her comments hurt you. Even if she gets angry or says something that hurts you or makes you angry, don't yell or make it a fight; keep calm and stay as mature as possible.
If you don't want to try this, or if you feel you can't try this alone, you mentioned seeing a therapist and that your therapist, as you wrote, got your mother to agree to let you see your boyfriend. Now, obviously I'm not saying your therapist can tell your mother to stop her lectures or to start liking your boyfriend, that's not his/her place. However, you can perhaps mention to your therapist what your mother has been saying and how it has been making you feel, and maybe set up a group session so that the three of you could talk about the issue and your therapist could help you tell your mother how she has been making you feel with her lectures and comments.
Good luck, I hope things work out for the best!
