How do you fix a stupid mistake...

About 10 years ago as a freshman in highschool I met a girl who went on to become my best friend for about 6 years. She ended up falling on some hard times and petty home so me and my sis invited her to lives with us cause she was like family. It was around this time I developed a relationship with another girl I had met. At first they got along well enough but after I told her that my friend was for lack of a better term easy, so my gf thought me and my bf were sleeping together but I assured her we never had. We go to my room one day and my gf sees my bf's shirt on my bed. I told her we shared a closet and she hung her shirts in there so she probably changed really quick and forgot it. My gf from that moment developed a serious hate for my bf after that moment. After a few months of there back and forth myspace message wars (remember those lol) it was to the point were I had to choose between the two. I choose my gf and broke off my friendship with my bf. Its been almost 4 years since then and my gf and I only lasted about 2 years but she's become my bf sand I care so much for her. But for the past few months I've been thinking about that a old friend and realizing how big of a mistake I made. I miss her so much and want to be friends with her again but if I do then I pretty much end my friendship with my ex/bf because she still carries so much hate for her even after all this time.