It's AU, Kurt and Blaine didn't date in high school and they were roommates in college. The fic takes place five or so years after college and Kurt shows up on Blaine's doorstep after dropping off the map with his boyfriend (I can't remember the name, it might have started with M though). So Kurt is extremely depressed because his boyfriend dumped him and Kurt gave up a lot to be with him so he basically has nothing now that the boyfriend has left. Blaine takes him in and they live together for a while. I remember they have sex once and Kurt starts crying because Blaine is so gentle and loving with him and his old boyfriend never was (no preparation etc.). In the end Kurt realizes he's in love with Blaine and leaves because he's afraid Blaine will reject him and he can't handle the heartbreak but he can't go through with it so Blaine has a chance to say he reciprocates the feelings and they live happily ever after.
Any help finding this fic will be much appreciated!
EDIT: FOUND link in comments.