I never thought I would be the home wrecker.
Are you really? How do you feel about being a homewrecker?
I am deeply sorry, I never meant for it to be this way.
Intentional or not, being a homewrecker causes a lot of drama. I am glad you feel very bad for what has happened and I hope it won't happen again. You should just end the relationship.
I care about her mom though, so this mistress (which is my best friend's girlfriend) loves me too so I have second thoughts about leaving.
Don't you think this situation is very sticky? If you and your mistress truly love each other, then perhaps it's best for him to break it off with his girlfriend. It's very unfortunate that things are turning this way with all of you, but I hope everything works out in the end.