Things my gf did and said before she broke up with me... does that mean she was seeing someone else ?

people dont just get engaged so suddenly. she was definitley seeing him while you guys were together if it was that she short after she got engaged. it would make no sense any other way. you do not deserve to be treated that way because you seem so nice and it seems as though she has lied to you. its not fair to you. i would say just move on because you know deep down that you deserve someone better than that. it will take a while to get over her but you have to do it for yourself. you cant get yourself involved with somebody is about to get married. so wish her the best of luck and move on, showing to her that you are the bigger person in this.