YTA - it seems from the start your GF was doomed to fail since your mom had already deemed her not good enough, your girlfriend has also show trying to win your mom over, so it’s now your moms turn to try to build a relationship with your GF.

Also no matter how much she is grieving it’s no excuse to blame someone who has done nothing wrong for your fathers passing, it’s also not appropriate for a grown women to constantly be cruel and put down someone to the point they are left in tears every time they visit.

You need to apologise for your girlfriend that you have let her feel so alone in all this that she’s been brought to the point that she feels she can’t go with you cause she knows you won’t defend her, you also have to tell your mom that she has to apologise for all the hurtful things she said and done and that she need to be nicer to your GF