In a situation like this, I suspect there's really nothing you can do. If your parents honestly believe that if you don't go to Mass every week you're hellbound, little is going to change their minds.

If they're willing to be remotely open-minded about it, the best advice I can offer is to articulate very clearly that you're doing this not to be a rebel, but to follow your own spiritual beliefs. I suspect you've already done this.

Challenging their logic (or lack thereof) is bound to lead to fights, but to a guy like me it sounds almost irresistible in your position. That question about your girlfriend was a completely valid one. If you're going further down that road, ask them about non-Catholic humanitarians and other non-Catholic upstanding members of society. Ask them if they genuinely belief each and every one of them is going to hell.

In all honesty, though, I'd put your odds at about 1 in 100 of making any headway on this situation. If you can't convince them to be more open-minded, evaluate your options: is going to Mass really that unbearable? Is there a way you could skip it without being caught? How much more money do you need before you can buy your own apartment and force your parents into an equally loving but less domineering position?

Best of luck to you, Jophus, and I'm truly sorry for your situation. As you might be able to infer from my avatar, I'd go crazy if I was in your shoes.

I'm not sure aboiut this, but maybe you're just at the wrong church. I mean, wah tyour mom said about going to hell if you don't attend mass, I don't think I've ever heard the word hell outside of the bible in my church. What i'm trying to say is that not all Catholics necessarily believe what your mom does.

Jophus:
Until you move out from your parent's house you are obliged to do what they ask you. While you may disagree with the catholic church (as I do), there is no harm in attending mass once a week, look at it as a social thing and something you do for your parents out of respect for them.
If they are devout catholics, nothing you say or do is going to change them or make them think differently about you going, so why bother agitating them?
I got into this same fight with my grandfather in California, I told him what good was going to mass if you don't act like a christian outside of church (my grandfather is a vocal racist) and all it did was put a large rift between us.
On a bonus note you may get to meet some cool catholic girls before or after mass, keep an eye on them as they are often much wilder than they would appear in church. If that's not motivation to go then I don't know what is.

Ask them why they don't believe in any other God, and when they give you an answer say thats the reason you dont believe in theirs.

Talk to them, peacefully. Share what you believe and don't about the Catholic church. Avoid bashing it in their faces. And oh, it's not a good idea to declare you've gone into, say, Satanism, while they don't even like you not going to church.