Hmmm, not an easy one ...
I guess she might be scared that if you become really close that she might loos a good friend. You seem to be pretty important person for her, as you have this very deep connection with each other, and frequent.
Well My answer will be: honesty, honesty, honesty. Just tell her how you feel about her, talk about yourself (and only about yourslef) absolutely open and frank and maybe ask her how does she feel/ what does sje think about what you have told her? I know that it is sometimes hard to talk openly and honestly, but trust me, once you did, you will be happy you did it. And maybe once you discover what her reasons for being scared are, you will know what and how to react. If you do not know what her reasons for getting scared and going back are, then just ask her. I do not think that you should use tactiques or any kind of extra seduction...you already know her so well and she knows you so well, and is definitely close to you (well not as close as you want...but)...
This is what I think. I hope you get more answer here too.
I wish you all the best.
Barb