It seems as if you guys may have started dating as teenagers and "are finally adults" meaning you just turned 18. Being 18 means you two have become of legal age, not that you are really adults if you both are not self-sufficient and still need assistance from your parents. That being mentioned, you've also disclosed that he still lives with his mother, which, in her mind, would give her the right to interfere into his life.
Realistically, you can't "get her off" your "back" in the given situation. In her mind you guys are still kids and she is still a mother-hen who is there to protect her son. Your boyfriend, her son, needs to stand up to her. If she's criticizing you and your family is should be apparent that she doesn't like you and/or doesn't feel you are good enough for her son.
You should take the high road in trying to be respectful of her, yet firm when it comes to making your decisions (if you're a good and prudent decision maker) or learn to evaluate whether you are making a good or bad decisions. Also, learn to have a thicker skin when it comes to matters concerning her, since you know she doesn't like you. It's hard, but if you intend to be with your boyfriend for a long time (even marriage), you will have to learn to live with his mother. If you begin to attack her or her character he will only begin to dislike you.