I'm not sure that I understand her behavior surrounding this, myself? It does seem hurtful that when you need and want her support that she's not giving it the way you desire. And seems more hurtful that it winds up in an argument. It doesn't make sense that supporting you is contingent on who you are involved with.

It does sound like she has reservations about him. Perhaps valid to her?

How long ago did you divorce? Are there children involved that would give her pause to be accepting? How long together have you been with your new partner? Does she need more time to adjust to his presence?

What about your father? Is he still in the picture? Did he work a lot of hours? How is/was their marriage?

Don't mean to appear prying with so many questions, I am trying to understand where your mother is coming from.