Forcesho,
Dump that golddigger. She's taking you for a big time ride. Find yourself a woman that supports you through school, pays every other time you go out, and would be satisfied with a .5 - 1.0 carat ring (even better no ring at all). I'm still trying to figure out why I dumped mine that fit that profile.
Women who want a big ring, just want something to brag about to all their materialistic brainwashed friends, and the ring will just be the beginning. That's not the type of woman you want. There's no woman worth that, no matter what they do for you... because you can always get whatever it is they do for you for much much cheaper. And if she is just so damn good looking, it's just a matter of time before she will cheat on you, or her looks will fade and you'll be left with a high maintenance wash out. In which case, do youself a favor... dump her. You'll either be happy to be rid of here (eventually), or she'll get a much needed attitude adjustment. Either way, you win.
Oh, and why in the world would you ever even think about buying a girlfriend a car? :Q You buy it and give it to her, it's hers my friend, even if she dumps you the next day. If she were your wife and the family needed another car, that's a different story. But a girlfriend, hells no!