Katherine
Good guesses. I can't be with her because of my curse. I was given a soul as punishment, to ensure that I would spend the rest of my life suffering for my sins. If I ever experience perfect happiness, I'll lose my soul and become evil again. It's happened before, when I was with Buffy. Her friend was able to get my soul back into my body, but not before I nearly destroyed the world. Buffy had to send me to a hell dimension to stop it. As long as my soul is only conditionally attached, I can't put Buffy at risk like that again. Spike is definitely obsessive, and the thought of them together makes me want to throw up, but at least he can give her some of the things I can't.
You're repaying the favor more than you realize by being my friend, actually. It's good for me to have people around. Helps keep the darkness at bay most of the time. I need to be a certain degree of miserable, but too much and I start doing things I regret.
Yeah, I've been to other dimensions before. Portals have been a surprisingly common part of my life the last few years. Any of those witches from your world show up here? If I'm not immune to this dimension's sunlight, that could be helpful.
Your secret is safe with me. I had no idea you used to be a vampire. How did you become human again? Do you have Mohra demons in your dimension? Some sort of Shanshu prophecy?
What is it you're trying to regain? Maybe I could help you find it.