› firstname.lastname@example.org 
I broke up with my boyfriend when he moved to another city after a short but intense relationship. Since then, we've visited each other regularly and continued having a sexual relationship. I've been fancying/dating/shagging other people for basically the whole time. But now he's started expressing interest in another girl and I'm jealous, though I'm trying not to let him know.
I don't want to get back together. Apart from the long-distance factor, every time I see him I'm reminded of all the ways in which we are incompatible. The sex is good, but not the best ever. Still, I spend a lot of time missing him, thinking about him, and feeling resentful of this new woman! Why can't I let go of this?
Oh, who knows. I think we're just mammals and once we've marked something or someone with our (insert yucky metaphor here), that something must forever remain at least partly ours. Yesterday my daughter claimed an empty kefir bottle and carried it around with her for hours, reclaiming it this morning the minute she saw it in the recycling. OK, she's a toddler, but I don't know how much we ever mature past the "Mine! You can't have it!" stage.
It's hard to let go, even when what you're hanging on to is entirely unsuitable. You don't really want that empty yogurt jug; you just don't want anyone else to claim it. But someone will, and you'll be fine. In other words, it's not that you can't let go, it's just that you haven't yet.
It should be obvious that the best way to get you past this in a hurry is to stop seeing the guy. You don't really want to be this dude's booty call, do you? And he doesn't seem like such a great pick to be your booty call, since he's only pretty good at the only thing you're likely to be doing together. It would be different if you were, say, an aspiring singer and he were the only accompanist who understands ... oh, never mind. This story is going nowhere, just like what's left of your relationship with Visit Guy. You miss him because you see him. And you resent the new girl because she's taking some of what you've got left. Give her the whole thing. You don't need it.
I am confused by my thoughts. I fantasize constantly about my wife having sex with other men. She's refused, so I quit asking her about it. But now I can't sleep. I am 36 with three children, and am having three or four wet dreams a week about my wife having sex with other men. Most of the time the men don't even have faces it's just me watching my wife have sex. I love my wife very much and wonder why I have this fantasy.
Nobody knows why, so don't look at me. I mean it nobody knows why anybody fantasizes about or fixates on anything. It's not just that nobody knows why you, the guy who wrote Andrea a letter, fixates on seeing his wife have sex with another guy. It may help to know that you have a great deal of company; indeed, accessing a little porn on the theme may help take some pressure off. Your search terms are probably "cuckold" (now that's a word with some years under its belt) and "hot wife" (although there's also "troilism" and "candaulism" if you want to get technical). If you do not want to see or read some porn, I suggest you never, under any circumstances, Google any of those terms. Even "wife" alone will get you some things you'd probably never want to tell your spouse that you've seen especially since you've asked her, been turned down, and she'd probably prefer not to have that particular discussion again.
While I was waiting for the coffee to kick in this morning and trying to force my brain to cough up the term "troilism" for you, I did a little search myself and found this quite nice article  on Nerve that goes into great detail about the culture that has grown up around cuckolding-the-fetish which is not the sort of thing that could have flourished in the pre-Internet age but has certainly come into its own. And aren't we proud?
Actually, I have no problem with it, provided it isn't one of those situations where the man (usually) begs and whines and cajoles and bothers the woman (usually) past the point where it makes sense to do so. Either she won't change her mind or she'll agree to it, meet someone else, and leave the first guy. Either way is OK with me. But neither of these things applies in your case. You're OK. If you can't sleep, try exercise, melatonin, or masturbation.
Andrea is home with the kids and going stir-crazy. Write her a letter! Ask her a question! Send her your tedious e-mail forwards! On second thought, don't do that. Just ask her a question.