Oh sweetheart, I am right there with you. I am driving myself crazy in my head right now and I am doing everything in my power to get it to stop.

Spockette is right, you need build a life outside of this girl. When those negative thoughts pop into your head, stop them, put them in a bubble and let them float away then think of all of the wonderful things you have done together and all of the love she shows you. (This "bubble therapy" has done me a world of good these past few weeks.)