Start stroking again. Do you want to cum while thinking of fucking her? Of kissing her? Of touching her breasts? That's not going to happen. You are not allowed to fuck her anymore and you shouldn't even be allowed to think about it. Only if you embrace her being with her new lover you are allowed to cum. You can leave and quit the therapy session at any time you want. But only if you finish it, you might be allowed to cum.
We will now focus on accepting, that she isn't yours anymore. But first give her another edge.