Plz Help!! sud i just leave him?!?!??!??!??! i love him..but...?

i really love him and he keeps telling me that he loves me and that he just made some dumb choices and really messed up, but what it amounts to is that hes w/another girl and he says that he cant just end it w/her that he wont do that to her, but he says that he still loves me and thinks about me even when hes with her... i tried to just leave him, but i missed him so much and he left me messages saying that he missed me a lot and that he really loves me and i just dont know what to do.....