Sounds to me like an infatuation turned obsession. I had that happen once. In fact, I still sometimes let my mind wander to the "good" memories we shared, before the obsession turned him away and I was left....well, confused and hurt and wondering what the hell happened.

Best advice I can offer is to keep busy and distract yourself whenever you catch your mind wandering to those memories. Remember to live in the present, and to make the present worth living in.