Cant stop thinkin about my ex!!?

i recently got engaged to a wonderful fella who thinks the world of me and my daughter frm a previous relationship, we have been together for near 2 yrs, my daughters dad was a fling that resulted in pregnancy, but although my fiance is great i cant stop thinkin about my ex bf frm school, we were with each other for 5 yrs on and off, mainly on! he was the love of my life, i think i loved him more than he loved me but i no he did love me, he was just immature at the time and liked goin out with his mates more, we had a short fling again wen my daughter was born but he ended up with another girl who he got pregnant, i see him alot, he is still with the other girl althoug he always talks about old times with me and sumtimes i think he still has feelings 4 me, its messin up my head everytime i see him and dont no where my heart lies, i dont want to wreck my relationship i have now cause i love my partner but i feel like im betrayin him by having these thoughts, what sould i do?