With the forced to choose between the two thing, you're kinda looking for Reverse NTR, A being your girlfriend, B, being you the person caught in the middle, and C, the netori, being your Ex. Choosing your ex is the betrayal.
Timestamps, kinda, you're in a long distance with a current girlfriend, and due to events in the game end up having opportunities with your ex... If you're looking for a reverse ntr vibe, where the ex is supposed to completely absorb your love, I don't think that's what you'll get from it. Also, fair warning, Timestamps has a huge mommy fetish that was enough to make me, a guy who likes incest, but prefers sisters, to stop playing.
You could also, kinda, count Parental Love, in which you were separated from your wife and 2 daughters for 16 years after dealing with a drug addiction and you're finally coming back into their lives, of course, your ex-wife is still in love with you, conveniently, and you also find that your oldest daughter is hot and has the hots for you. To add fuel to this fire, your sister who you'd apparently been fucking while you were apart, effectively cuckolding her husband, also comes to town, stays with you all, and is a generally sexual influence on the game. Again, not so much the NTR vibe, though there are options that might give some of that if it's what you're looking for.
Sadly most of the games I play the MC was either a virgin, single, or just got out of the relationship with the ex and so they're in a state of hatred, not possibly going back. I can't think of many games that do this, or even many others that have the reverse NTR vibe, since they all want you to be able to fuck-em-all.
