My personal feeling on this, based on the information that you have given, is that to marry her on her the grounds that she won't speak to or see your mother if you don't would be a huge mistake. What kind of a message would that give your girlfriend? That by using blackmail she can manipulate you into doing whatever she wants? That's no basis for any relationship let alone a marriage. Apart from that it would appear that she really doesn't like your mother anyway, so there's always the possibility that you will marry her and she still won't talk to her! Even if you do get married you can't force her to talk to your Mum just because she has a ring on her finger! If you truly love someone you don't blackmail them into marrying you surely? Where's the respect in that? To give in to her on this would be giving out all the wrong signals and telling her that you are willing to be treated that way. That doesn't bode well for your future happiness. She may be trying to give you a kick up the backside to progress your relationship but in my opinion that's certainly no way to go about it. Marrying under threat? I most definitely wouldn't! Apart from all that, you say you don't know why you can't make a decision. I think I do, you are worried about making the 'right' decision. The right thing is to trust your instinct, be true to yourself and do what is right for YOU.
I wish you all the best whatever your final decision.