Two of my best friends have had a long-term on-again, off-again fling/relationship. They’ve both told me things that make it clear they shouldn’t be together. I’ve advised them to end their relationship because they’re often unhappy. They called it off a while ago and said they’d try to remain friends.

I also recently ended a relationship, and a few nights ago, my guy friend came over (from the aforementioned couple) to hang out. I used to have a HUGE crush on him – before he and my friend started dating. That night, we talked about our failed relationships and wound up hooking up. We both agreed it was a huge mistake, and we both feel horrible.

Two days later, my girlfriend told me that the two of them are still talking and that she still has feelings for him. That made me feel even worse because I hooked up with a guy she’s really into. I plan on telling her – I just don’t know how. I always told myself I wasn’t the kind of friend who does this, but I did. I don’t want this to define what kind of person I am. How do I tell her? And what role can I play in their lives and relationship now?

– Meredith