Can't get over...

So I was with my bf/fiance for about 5-6yrs we were even engaged. Well one day he just up & left me no explanation NOTHING! Well here to find out a week after we split he was already with someone. So that did me in & hurt me. I blocked him did everything I could, but he ALWAYS found a way to still talk to me & message me. We needless to say he talked to me the whole year we were split & he was with her. Long story short they split & now we are back together ❤ 2yrs strong & better then ever. BUT I still can't put her in my past & still think about that just hearing her name makes me sick!! How can I get over this!?! His old friend told me that I'm stuiped for taking him back because he'll never be over her, that he loved her more then he ever loved me & that ALWAYS plays in my head! So my question is how can I get past this!?! Like she never even happened!