I don't think it makes much sense to be guilty about anything that has happened...you've been clear...you probably really want to work on why you'd feel guilty, guilt should be reserved for when you're doing something wrong you know?
But it also doesn't make much sense to expend too much energy on somebody who seems to want to be monogamous, and that you can't see because of distance. Yes I'd say let them go, there are lots of amazing people out there in the world, that will suit both of you better as partners most likely. Poly is difficult enough without thinking (before there is really actually much of a relationship) that you wish you could be split in two. I'd suggest starting off dating people who are already poly or in open relationships and used to sharing their time and energy. That's great she is open minded enough to give it a try but...if she isn't going to go out and actively date now, I don't really see this as anything more than an exercise in misery. "Like each other a lot" is a much better place to stop than "We're in love" and going to be tortured, and this situation is going to negatively affect her, and my wife, and me etc etc.
Is your wife dating other people? Have you asked her how she keeps from feeling guilty and not feeling like she should break up with you to go spend 100% of her energy on another partner?
I have no guilt about engaging with new partners because I tell them my situation and what that means for time available/what a relationship can be, ranging from FWB up to co-primary relationships depending on your agreements, and that's all you can really do. If you tell them that you are married, they won't expect to marry you. If you and your wife are open to having co-spouses at some point, you can let people know you are open to that. If you are only available X amount of time, then you let them know that, and you stick to it so they don't think you can be sweet talked into changing your boundaries. If you don't have money to travel to see a LDR, you are upfront with that, and then nobody expects that the relationship is going to be in person except for rarely.
Really with the amount of guilt you are feeling in this situation (I don't understand why you are feeling so much misery about it, do you?) I would suggest not engaging with them anymore, and working on why you felt this way (with your wife, yourself, friends, anonymous online forums) before dating at all, and then I'd stick to locals if possible, ones who are either seasoned at non-monogamy OR are open to trying it but don't have a known strong desire to go have the mono one and only at some point.
Happiness will never come to those who fail to appreciate what they already have.