cant get over bf past

so its the first time im posting here think i really need advise. So my bf claims he's attracted to older women and i had no problem with that peopke have there type and preferences. i'm 20 though and he prefers women in there 50's and so on. So i recently found out that he was sleeping with one of my lectures she is like trice my age, old and unattractive well to me.( nor judging just being honest) i cant seem to get over it even though it was before me. like the woman taught me for heaven sake and u were with her. i keep imagining them going at it i cant remove it from my mind , like guys how do i accept it as his past and put it behind me its destroying my relationship.???