You're in a tough situation. You're right to be very concerned, before permanently committing to this young woman. I have to wonder: How were things going for most of the last 4 years - up until a few months ago? Was this never a problem before lately?

If, after 4 years, your mom and gf don't really like each other, that's probably never going to change. The rule is: They are socially obligated to be courteous to each other. They do not have to love each other, or even like each other. But your mother has an obligation to be respectful to a young woman you are seriously involved with. Your gf has a duty to be polite and respectful toward your mom. If your gf is declaring that she just isn't willing to be around your mother, then maybe you do need to consider ending the relationship. She cannot ask you to choose between her and you mom. A family is kind of a package-deal.

It's very hard for two people to have a fight, if one person refuses to fight. Older people get set in their ways. I think your girlfriend needs to be flexible. After marriage, your wife becomes the #1 person in your life. You need to reassure your gf of that. Knowing that, she can afford to be generous and show some tolerance of your mom's ways. That does not mean you expect her to put up with abuse. If your mom really mistreats her, then she has a right to alert you to that. Then you can make an excuse that she needs to leave.

You have to be alert and pay attention to what's going on. Who is being mean to who? If your mom is really mistreating your gf, then it's your job to explain that you and your gf will have to step out and leave, if mom can't be fair to gf.

It may be unrealistic to expect your gf to agree to live in your mom's house after you and she marry.

You and your gf have much to talk about. Listen carefully to her. Reassure her of the place that a wife would have in your life. Then communicate what you what you would expect a wife to tolerate from your mother.