You won’t get much sympathy here OP. You have behaved appallingly.
But
It does take unexpectedly living through this kind of thing to know how horrendous it is. I never intended for it to happen in my life either. You can hate and judge yourself more than anyone else can.
You are just a person though. This kind of thing happens to people. There's a whole body of literature, art and music to testament to that. These are the emotions that thrill us.
It’s obviously better to strive to not be in this position, but you are not alone. And the hurt you feel is real too, although it’ll never be socially acknowledged, and definitely not in AIBu
What I'm trying to say is don't be so down on yourself that you depress yourself out of action. You need to act with honesty and bravery to get yourself out of this situation before it gets worse and a broken you begins a pattern of behaviour within this obviously broken relationship. Because if you don’t address this, you’ll do it again.
You are a member of a species that has the capacity to be amazing, unselfish, strong, brave . Strive for the highest. Lift yourself clear, give yourself space to be sort this out, give your partner to chance to move on.
My advice would be finish both relationships. And Be alone for a while . Get counselling. Find out why this has happened . There will be a reason in you that has given this precedence. You will find it if you look, but you need to clear the slate, make amends and then be alone to find it. Give yourself a chance. Because even if you get thru this with one of them still loving you, you won't ever love yourself .
A good start would be the relationships board rather than AIBU, there are posters there who have compassion, empathy and knowledge of the kind of emotional landscapes, scripts and backgrounds that set the scene to this kind of situation. You won’t get much sympathy, but you might get compassion and knowledge.
Good luck x