(Newbie speaking, take with grains of salt).
I don't understand your therapist's perspective on this, to be honest. Why would the depth and intensity of your feelings for a person have anything to do with your sexual orientation?
You describe your current hetero relationship as "wonderful". That (to me) indicates that whatever your orientation is, it does not preclude heterosexual relationships, which means you're not strictly lesbian, you're bisexual or you fall somewhere on that spectrum, depending on how you look at it/which terms you prefer.
I'm sorry things aren't going well with your new female interest, though. I would venture a guess (solely based on my own experiences, I'm probably inappropriately generalizing here) and say that yes, the fact that you have never been with a girl before is probably fueling your interest. I had an interest in a very fascinating lesbian girl once, years ago, and I thought she was interested in me, too. She wound up flaking out on me, too, and I guess she wasn't interested in anything remotely serious. I had the same feelings of being broken up with, but being in a good relationship at the same time, and my boyfriend (now fiance) didn't really know how to respond. That situation was part of a messy series of situations that corresponded with a long period of breaking up and getting back together, which I am now thinking might have been avoidable if we were better at communicating and had heard about polyamory. We did end up pulling through it, though, and I think those difficult situations helped us to develop the better communication skills we have now.
I guess that wasn't really all that helpful in the practical sense, but I just wanted to at least throw out some "I hear you!" and "I can relate!" vibes.