why was i so dumb?

so at the beginning of the school year i was talking to this guy and we never officially dated because i had a feeling he wasn't for me, he had been with a ton of girls and he was nice and all but just seemed a little sketchy. we don't talk to eachother anymore but today i was thinking and i remembered him always talking about his ex, how she was his first kiss and all. and i was like ? why are you telling me this? and he would send me pics of them together and it made me sad but i never told him. i broke off what we had and the NEXT DAY. he had a girlfriend. i was walking outside after school and he knew i was right behind him (the day after i broke things off) and he was talking all lovey dovey to her. i know i should be happy for him but he obviously wasn't interested in me when we were together if he could get over me so quickly. also, he broke up with that girl and got with another girl very shortly after. i know i will not be talking to him again :)