The emotional pain could be secondary too; the guilt and shame about having the fetish. As well as a conflict with the social norms of monogamy. You're not 'supposed' to enjoy your girlfriend having sex with another man and it doesn't into the rules of monogamous relationships. Can you accept what turns you on, and seek out relationships where you can act out these fantasies? e.g. open relationships, swinging, polyamory. Or you might find a woman who agrees to roleplay the fantasy through dirtytalk without actually doing it, or exchange nudes with random strangers so that you can witness her doing that. So there are different levels you could express the fantasy without ruining your relationships.

I became pretty obsessed with cuckold porn around the time I was in a relationship with an abusive woman. She broke up with me several times, saying she cheated on me, then taking it back and saying that was a lie, repeating that pattern, saying she lost attraction to me and didn't like having sex with me, etc. I think I sexualised something that was very emotionally painful, as a way to distance myself from the pain of a situation I felt unable to escape from. Like you I also felt horrible and sick about it, and simultanously turned on. Which is a disturbing experience. So I understand that part of the emotional pain.

But now that I've been single for ages, and started healing from that relationship, I don't have a strong interest in cuckold. It's just one of many things that turn me on and I don't feel too weird about it.

Part of my interest in cuckold is because I'm bisexual so there's a grey area in my fantasies between classic cuckold situations, and merely bisexual threesomes with some element of domination/submission. Some cuckold porn is almost an equal threesome where one guy is a bit more submissive than the other. I also just like to watch groupsex porn featuring 'cuckold positions' regardless of the people or dynamics involved.