Perhaps you can find a way to feel 100% committed and loyal and have more than one love or lover at the same time? Part of my emerging clarity is to realize that what I have yearned for in intimate relationship has been honesty and transparency, that these qualities are where I am able to make a vow for 100% commitment. I am so habituated to believing that the left hand should not, cannot, must not know what the right hand is doing, because of my own acculturation and my own personal history of being a liar and cheat, that now when I feel attraction or romantic feelings for more than one I do have to take a few steps back and let go of those knee jerk reactions. The impulse is very strong in me to squelch my heart and/or hide my feelings (from myself).
Perhaps you could actually explain your dilemma to both your SO and this new person?
I have found increased communication to be the reliable solution. Also, radical acceptance of my feelings and what is going on for me. I said to a woman the other night "I am attracted to you but my mind is telling me it's a bad idea because you have a SO and I'm separated from my most recent SO." She was relieved to have a minute to talk about things and said "I'm attracted to you too but you seem like a real dangerous character right now and my boyfriend is monogamous, so let's just take it easy." These are simple little exchanges that I would not have dreamed of having in the past.
Radical clarity has a relatively small price and a huge ROI, IMO.