On the top of my head I can only think of that one :
http://www.mangago.me/read-manga/jibun_katte/
I mean, if you mean the mc, and not the mc's love interest.
And that one too :
http://www.mangago.me/read-manga/kore_wa_kitto_yume_no_naka_no_kiss/
I put together stories I like that has that kind of plot here http://www.mangago.me/home/mangalist/43415/
Does anyone knows a shounen ai or yaoi where the mc used to date girls or are in a relationship but they are slowly falling in love with someone else (a guy)