Broke up with her behind this issue 2 months ago, went out with my best friend as his wingman to try and lift my spirit.
Ended up clicking with the friend of the woman he was talking to, real cool lawyer girl. We ended up fukking within 24 hours and the sex was great, amazing even. Me and her have crazy sexual chemistry, not gonna lie I was shell-shocked because I realize I hadn't had actual "great" sex in 4+ years.
... Then I got back with my GF 4 days later, thinking that I got the sexual tension out my system enough to go another stretch trying to work it out with her.
After an average sexual encounter and flat out bad sexual encounter that had us both seriously questioning if we could ever fix it. I ended up cheating and fukking the lawyer girl again, we're fukking til this day.
I'm still with my GF and I didn't tell her about the other woman, but I didn't want to be fukking both of them at the same time so I basically told her that I don't think sex adds anything positive to our relationship at this point and we should abstain from it for a good while and see how things go. It's been pretty smooth for the most part except a few nights where I guess she found herself being horny and would try to initiate sex, but I would just side step those advances and she wouldn't really trip or beg.
I suggested us abstaining for my own selfish reasons, but in a weird turn of events I honestly think explicitly cutting sex out of our relationship dynamic has made her way happier. She even noticed how good she has felt in the relationship in the past few months, but she was in denial about it being tied to us not having sex.
With the other woman she knows my GF exist and knows we're close, but since we met during the break up and never told her other wise, she thinks we're still broken up. And I know what I'm doing is wrong because I can tell she's catching deeper and deeper feeling as time goes on.

You asking for all kinds of problems, jump ship before its too late.
Aint no point in being in a "relationship" with two people at once.