OP, I'm so sorry you have to deal with this. It's a really tough situation that I wouldn't wish on anyone.

I have similar issues with my parents and religion -- raised in a religious household, attended Jewish schools and summer camps, and repeatedly told that the only acceptable SO is a Jewish SO.

By the time I was 18 I discovered religion was not for me and I've only dated non-Jewish men. Slowly but surely I was able to break my parents down and convince them that my happiness is more important than dating within the faith. I was too scared to even tell them about my first boyfriend in freshman year; I told them about my second boyfriend in senior year but they refused to meet him; they eventually met my third boyfriend during grad school; now they're looking forward to meeting my current SO.

My advice to you is to tell her about your GF (if necessary) and hold your ground once shit inevitably hits the fan. Your mom's overbearing personality will be tough to deal with (constant passive-aggressive comments about you and your GF) but at the end of the day you can remember two things: 1) your mom probably loves you and wants you to be happy, so there's a chance she'll come around and support you and your GF 2) even if your mom doesn't care about your happiness, she will want you in her life so she can continue to manipulate you. So, if you do something that she disapproves of, there's a chance she will continue to support you financially in order to keep you under her control. If that financial support is really what you're after (and you're willing to sacrifice your independence to get it) then things might still work out in your favour if you tell her about your GF.

Best of luck managing the situation, OP