I've been sleeping with my colleague. Neither her husband nor my girlfriend have any idea.
Why are you both cheating on your partners?
I don't know. It just ended up like this.
It is wrong to cheat on your partner. You are supposed to be faithful to your partners.
I agree. But it has already been so long like this.
The time does not matter, you need to end this now and tell you girlfriend what you have been doing. Keeping secrets like this will end badly for everyone involved.