Hi Toodles,
I'd like to pick up on two things. First is that, as I see it, you don't have an innocent friendship. I mean this in the sense that your friendship stands to hurt you, by keeping you enmeshed with someone who, for whatever reason, is making herself unavailable to you. And stands to hurt her, by putting her in prolonged contact with you when she has apparently decided to stay with her spouse despite her feelings for you. And stands to hurt her husband, who has to stand by and watch her maintain emotional intimacy with someone who is in love with her, and with whom she is apparently in love. There's not a lot of innocence here, in an emotional sense - only confusion, and pain.
I'd also like to pick up on your comment that you'll never love anyone again, not like this. I'm sure that's exactly how it feels, but logically you simply can't know that to be true. Have faith. Also, given how much pain this love is causing you, surely you'd never WANT to feel love exactly like this again! Wouldn't it be refreshing to find a person who sparked this kind of passion, but who was also responsive, available, and drama-free? You deserve the love without the pain.
I believe you can have that, but I believe it means letting go of your friendship with your former therapist. It will be painful, and you will grieve, but I think it needs to happen.
I also get a vibe from your post that you are in deep, deep agony and that maybe you could benefit from a healthy, un-complicated theraputic relationship. Perhaps you could find a therapist who would be utterly unappealing to you from a relationship standpoint (because of their gender, for example) and start from there.
If therapy seems over the top, I'd at very least consider talking to your doctor to rule out depression. Your post was heart-wrenching, and I worry for you.
I hope you find what you need, whatever you decide to do.
All the best,
Maitri