First of all, rest assured that you're not alone. This is extremely common, especially between a woman and her mother-in-law.
Secondly, I'm not surprised that you feel resentful. Your boyfriend clearly feels torn (and I feel for him for that reason), but he should really be making more of an effort to stand up for you regardless.
Got a broken heart, relationship niggle, infuriating family member, or anything in between? Email your questions here and check back next Friday to hear Jill's wisdom.
That said, it's not as easy as simply instructing him to tell his mother to back off. If it were, he probably would have done so already.