Hi, I'd like to help you with your question.
It sounds like your mother made the assumption that when she moved in with you that she would have your full attention. And when your boyfriend came along, she became jealous and started acting out her resentment.
Your mother's retirement has left her with no focus. So she is sensitive to the loss she has had with ending her job, which probably gave her more than she realized. It provided her with socialization and goals. Now she does not have those things. She naturally turned to you, maybe thinking that you both would spend time together and her loss would be filled with her relationship with you.
Your mother is going to remain resentful until she decides to change her situation. You can help by suggesting that she fill her time in satisfying ways. She can get a part time job, develop hobbies, join groups (book clubs, classes, etc) and start to develop relationships with other people her age. She can also volunteer. Volunteering offers flexibility in her schedule with the feeling of being needed and contributing to something worthwhile. It would help raise her self esteem, get her back out into the world and help her develop friendships.
You may want to approach your mother by talking to her about her job and her current situation. Suggest she try some of the above ideas. You can even offer to go with her for the first few times she tries something. Also, consider making one day a week a special day where just the two of you have fun. You may find that after a while, her attitude towards your boyfriend softens.
I hope this has helped you,Kate