Please help...

Fellow ents, I have an issues, it's about love. Theres this girl, she's madly in love with me, and i really like her back, you could say were boyfriend and girlfriend, we've been dating for about 2 weeks. My ex just recently started talking to me after a rough break up, needless to say this complicates everything, I'm still madly in love with my ex and shes madly in love with me, I just don't know what to do. I want to be with my ex but I don't want to end things with this new girl. I feel like I need to tell one of them that it's not going to work out, but I just have no clue. Someone please help me out, I just want to know what to do.