Nightmares are release dreams (for healing purposes) - it seems horrible but if you didn't work through traumatic stuff in nightmares, you would go clinically mad.
Also, I believe that a couple, separated or not, can be psychically connected to each other; for instance, I have often dreamt of Andy and something he will say to me and then he turns round and says it for real, or if we have been abstinant for a while, I will dream of ML with him and then it happens in the same circumstances as in the dream. Certainly, while he was living with EX-OW1, I dreamt of her quite a lot (even before I knew she existed and then I met her and it was the same face as in the dream), because you're like tuning into your estranged partner's surroundings etc so you're picking this up in the dream. When she left, I stopped dreaming of her and I've only had 2 dreams with her in, in the last year, and then in a very abstract sense. With EX-OW2 I only dreamt of her a couple of times in total and that was always in relation to him choosing me over her (which he did do) and I couldn't actually see her face so I knew it was a casual fling and not important in the scale of things.
It could be that now she's moved away from the OP, that is why you have stopped having the nightmares, because the OP is not in her surroundings, and it could also be that time is passing so you are naturally healing from the pain and therefore getting less nightmares.
