I doubt very much that the perpetrator is a succubus or an incubus, considering there was no seduction, or intimacy involved in this whole debocle.
It's possible that what you are suffering from may be an embodiment of shadows and demons (personal demons, not demonology demons).
An embodiment would adapt it's presence in your dreams to your updating affairs in the waking world.
That would also explain the 'formlessness' of this thing, and the lack of defining characteristics.
However, that's irrelevant-onto solutions.
I'd recommend casting some dreamtime wards, they can help you intern your nightmares by partitioning them away.
Or just do a banishing ritual, nightmare embodiments are, in a sense, conscious, and because of that, are technically another being within you-which means a ward against etherial intruders should dispel them just the same as it would, say, an irate spirit.
If you're really feeling brave you can get someone to help you detach your waking mind and allow it to drift among your dreams. Or even get someone to bridge your minds to help you contend with this evasive entity.
But that'd be overcomplicating it-doesn't matter what it is, it can be banished or warded against. Just do a simple warding/shielding/protection/countermeasure spell or two, maybe invest in a protective sigil or two, and you'll be good as new.