Ummm.....sounds to me like she IS crazy and annoying.
Anyway, I think you've done exactly the right thing. Giving constant reassurance to someone who's driving you nuts only reinforces their behaviour. Responding with how you are fed up with this behaviour is a much better reaction.
If I were in your shoes, I'd tell her that most of the time I really enjoy her company. But when she gets neurotic like this, that she does my head in. I'd say that I don't enjoy listening to her go on with this crap and that if she keeps it up it will ultimately drive me away.
You also need to let her know that doing things to purposely upset you is unacceptable and that if it keeps up, you will leave her.
These are not to be used as threats. You must mean it and be prepared to follow through....otherwise she'll walk over you forever.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.