How do I get over him?

So I had met this guy that I recognised on instagram one day (we had mutual friends) but we’d never actually talked before so after a brief convo I later messaged him on insta and then we started talking from there. Later things moved on and long story short we had been sending nudes back and forth for 3 months and we had never established what we were (which is stupid I know) and then yesterday when I was hinting at finally meeting up he said he wanted a more serious relationship (after constantly saying he didn’t want a relationship with anyone) and that having a relationship with me now ‘wouldn’t feel right after what we did’ I just don’t know what happened I knew that he was kind of a fuckboy but I just hoped he was different and now I’m just trying to get over him but I cant stop thinking about him and i just don’t understand what changed? Any advice?