I really screwed things up ...

Ok I have really screwed things up with me & my best friend. Only she doesn't know it yet. My best friend had been seeing this guy for the last 2 months, but they broke up 2 nights ago. While everything that went wrong with them was not all entirely her fault, she did have her share in ruining things with them. She got clingy way too fast & she would follow him around all the time almost to the stalker type, so this all really freaked him out. About a month into their relationship he & I started talking about how things were not good with them, but he was just scared to tell her. But he & I continued to talk; we just never mentioned it to her. And it was just talking, because at that time I was seeing his friend. So Saturday he called me to tell me that he was fixing to break things off with her & then proceeded to ask me to come see him. I said no because I didn't think that it would be appropriate. So of course this devastated my friend. So Sunday he called me to come see him and for some reason I agreed. When I got there we started talking about everything that had happened with them and some how we ended up sleeping together. I know it was an awful thing to do. And we both feel really bad about it. I feel like I am the worst friend ever and in reality I am. Because I know that I would hate it if she had done that to me. My thing is should I tell my friend about it and upset her even more or should I just go on like nothing has happened. Because I don't want to lose my friend, but I know that there is a good chance of it because of my stupid mistake. But I really would appreciate any insight on this! Thanks!