Need advice concerning an ex and a prospective girlfriend

This story starts several months ago. I was in a dysfunctional relationship, and began to have feelings for another girl. Needless to say, my girlfriend was upset, and cut things off. Feeling terribly guilty, I arrived in school the next day to find my ex crying. I talked to her for some time, and took her back, I felt as though I hadn't given the relationship a fair chance the first time around, and it had been completely unfair for her. We dated for several more months, and then went our separate ways. Since then I have been talking to the girl that I had been before. We talked frequently and things were going well. I even began to form a friendship with the Ex. Several weeks had passed, with frequent phone conversations between myself and this girl, and we hung out on a few occasions. But then one night I slept with my Ex, who I find that I still have some feelings for, and her for me. Now I don't know what to do. Part of me feels that I must tell this other girl about my Ex because she has feelings for me. The other part of me feels that I am single, and am within my right to associate with who I choose, although I need to make it clear to both girls, that I do not intend to rush into any form of relationship.