I hope you talk to guy-on-the-side about your feelings at least once before things split up. Maybe he cares for you too but doesn't consider something ongoing a possibility. I know that the idea is probably hard and scary. You could always just say something like "I hope you find someone who can give you all the things you want. The truth is, though, that I care about you a great deal and I know it'll hurt when we're not seeing each other any more. I've done some reading and it's possible to maintain multiple relationships ethically, but I don't know if you'd consider that a possibility, if you found a girl who was open to you having someone on the side, the way my husband is open to me being with you. I don't expected anything, but would you be interested in reading an article or two if I sent them to you?"
And yeah, if you want more feedback, definitely start a thread in the new to poly section. Good luck!
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.