Are you absolutely certain she's a "Succubus"? Could she be a disembodied Soul who has formed a strong loving attachment to you....or maybe has always had a strong loving relationship with you?
Such things can and do happen. Sometimes that is even a Soul you know. Maybe someone who is 'waiting' for you in Spirit, has either no wish to incarnate on Earth, or who hasn't done so yet. It can be a Soul you have always known, with whom you have a powerful bond.
Okay....so I'm not knowledgeable enough about "Demons" and how they operate, but to class this woman/Soul as one seems to not quite fit from what I have heard.
Her loving (yes loving because that's what they seem to be) interactions with you, her happiness to allow you freedom to experience your life with no obsession from her part, her kindness towards you...(and you have known her for some time) seem more like a Spirit who loves you, to me at any rate.
Also, the lack of negative side-effects -as you report anyway....the inspiration this relationship provides you....the spiritual growth it seems to help you with.....
I often think of the situation when one partner/loved one stays in the Spirit World, and one incarnates. It happens. And there are times when the other may return to incarnation....possibly later in that lifetime. But not necessarily. Incarnation isn't always a given. It always depends on what greatest benefit to both Souls can be achieved.
Somehow, this doesn't quite sound like a "Succubus" (?)
Yes some 'dark' Spirits will get on your good side, show you amazing stuff, amazing 'love', amazing sex, apparent loyalty, and roll you along with that until you are theirs. Then they will turn, just when you least expect it and when they have your full trust, and there is no longer any getting away energetically. And the cleverer and meaner they are, the more they string you along.
But she doesn't sound like this either....