I can't believe I cheated on my girlfriend with my best friend's girlfriend.
That's bad. Why would you cheat with your friend's girlfriend?
Because we share many interests and we spent time together frequently and we even hang out alone, so we think we should be a couple.
If you want to get together, you should stop cheating and break up with your partners, and apologize to your girlfriend and your friend.
It is already done so they will break up with us anyways.
Hopefully the two of you apologized and were honest about what happened so they don't think it went on longer than it did.