My son was in turmoil a few years ago and I asked him if he was gay. The relief on his face at that question will always stay with me. I don't have a problem at all with his sexuality. But his father doesn't know. And we are not going to tell him. He has voiced his homophobia on many occasions.
Neither of us is keen to 'test the water' with him, so it's left. The danger obviously, is that he, and I, may face big problems if we do tell him and he does not accept him.
I don't like the situation. But I don't have a crystal ball to judge the outcome if we tell him. I dream of the day he knows and it isn't the problem I fear.
So, he has one parent who accepts, like you. But is spared the difficulties of having one parent who doesn't.
I know we cannot make my hubby accept if he doesn't feel he can. And I can't think of anything you can do to make your mum change her feelings.
Perhaps in time, with help from your lovely dad, it will change. In the mean time worry about yourself and your adjustment, and be very thankful for your dad. And keep a friend of your mum if you can. I'm sure she can't help her feelings, and wishes like you she can accept.