How do i get him to want me, more than her.?

so me and this boy zac met at work. before we even talked we would see eachother and flirt from afar whenever we we around eachother. a few weeks ago i went to a party with people from work and he was there. we immediatly started talking and hung out all night. we got drunk and made out, then he came home with me before the cops came and busted the party (which really sucked eh, =/) but anyway we went abck to my friends house, and i never have with him or anything. but we went to sleep and kissed/cuddled all night. so then i came abck to work and everyone totally hated me because everyone wanted me to date this other guy that i had been talking to. i didn't want to deal with it, so i got rid of zac and started talking to andrew. well turns out andrew was not my cup of tea so i recently started talking to zac again but he told me he's also talking to this other girl now. he already told me i was way prettier then her, but has bummed her off before and told her he wont do it again.