im sort of seeing my exes best friend but noew my ex wants me back

Hello again, thank every one for previous advice it helped a lot. But now I have an even bigger problem. My girlfriend dumped me and I tried so hard to get her back but she kept sayng no and ignoring me, I started getting closer to her best friend because she understood me and felt for me because of what my ex did.. One thing led to another now I'm sort of seeing her, we're not officially going out but I thinks she wants to because she says she really likes me. But my ex started acting jealous when she first thought something was going on and said I was only seeing her friend to get back at her- which wasn't true at all! Anyway she eventually says she wants to talk, and eventually I ask if she still cares- she says yes. I loved her more than anyone and was willing to give it another go, so I made the effort to speak to her and tell her I still care too. But she then tells her friend this and her friend says that we are just friends.... So my ex is happy again and back to wanting nothing again. But her friend still likes me and we start seeing each other again, I do genuinly like her. But last night my ex starts acting jealous again this time she tlls me she doesn't want her friend to have what she so stupidly threw away. I don't know what to do because I really do like her friend but... I love her. I'm stuck because I know its not fair on anyone to start a relationship that would just be a lie and hurt people. Any suggestions? Please and thanks...