<human>: So about a year ago I broke up with my girlfriend of about 1,5 years after realising that I had fallen in love with her sister. I tried to suppress my feelings and work on our relationship, but eventually I gave up and left her. I didn't tell anyone why I did it; my "official" reason is that I just didn't feel the same way about my girlfriend anymore. It was very painful to lie to her because I do care for her a lot, but I couldn't bring myself to break her heart like that and potentially ruin her relationship with her sister.

I thought I'd get over this infatuation with the sister over time; it turns out I thought wrong. I'm still constantly thinking about her several months later and because of this I generally feel depressed, pessimistic, and unmotivated. I cannot find joy in anything I do; I can't enjoy games, reading, films, sports etc. I find myself not wanting to meet my friends or talk to people in general. I also suffer from insomnia; I can't fall asleep because I'm thinking about her all the time. I've basically locked myself in self-pity and I don't know how to get out. I've tried to meet other girls, but it always feels like I'm "cheating" on her (the sister), so I always end up not even giving it a shot.

What should I do in this situation? Should I tell her my feelings so maybe I can clear my head? Should I talk to my ex? (We're still friends.) Should I just man up, move on and forget it all?

Could you distill the essence of the text into 1-3 sentences?
<bot>: Broke up with girlfriend because I fell in love with her younger sister; didn't tell anyone. Several months later, the feelings are still there and I feel depressed. What do?