@mike02526,
So many of these requests for advice contain the statement:
I love my (
girlfriend, boyfriend, husband, wife)
but...
It's always impossible to know just what someone means by "love" when they say this, but it's often the case that I'm left with the impression that "love" doesn't mean to them what it means to me.
If I take it for granted that you sincerely love your girlfriend than I would suggest that seriously consider what she is requesting, not in the sense of her specific demands, but their substance: She wants to marry you and she wants to have children. I can understand that you may not want these things on her timeline, but that doesn't mean they have to be on your timeline. Having kids is a big deal and not to be taken lightly, but it's hard for me to imagine someone sincerely loving someone else and not wanting to be in a committed relationship. If that's what your balking about I doubt the sincerity of your love. If it's just the marriage thing, what's your reluctance due to?
I'm afraid though that I can't take it for granted that you sincerely love your girlfriend, or, least, feel what I would call love for her. If you did, I would expect your quandary would be limited to not wanting to break her heart, not what spiteful vengeance she will exact when you do.
With this being the case, my advice is to end the relationship now, as you're not in a position to give he what she wants, even sometime down the road. That you are seriously worried about her wrecking your car or getting you fired if you tell her no (let alone break her heart) suggests she's probably not someone you should marry and have kids with. So it's a win-win for you both.
Avoiding having your car wrecked seems a simple problem to solve. Be prepared to beat a fast retreat once you tell her no. As for your job, I'd have to know why you think she can get you fired before giving you advice on that.