Not interested anymore - how do I break it to her?

For the past few weeks I've been in a weird friends-with-benefits type relationship with one of my friends, who's a trans girl. Since both of us are underage, we haven't actually had sex - it mostly just involves flirting over Skype and feeling each other up sometimes irl. The reason we both decided to do it is because she finds me attractive and we're both into some kinky stuff that we thought it would be a good idea to explore together. Just to be clear on this from the start, she has done nothing wrong and everything we have done has been consensual. She's been a good friend and very supportive the whole time.

The trouble is that I'm just not as interested as she is. And I don't know why. I've always been reluctant and nervous about it but she's been encouraging me through it while also knowing when to back off. It's got nothing to do with the fact that she's physically a male because I always just imagine her as female when we talk online or do stuff. I've never been particularly squeamish about sexual stuff before this, so...what's the problem?

I've been out of touch with her for the past few weeks and that made me realize that I don't really want to do this stuff anymore. Whenever I can talk to her on Skype the conversation always ends in dirty talk when I just want us to talk like normal friends like we used to, and while a few times I've liked her touching and complimenting me irl, other times it felt like an obligation or just downright awkward and uncomfortable. Whenever she wants to try roleplay over text chat I always get really nervous about it, and I told her that once but she tried to psychoanalyze me and find the root cause of the whole thing which really didn't work, so now I make excuses whenever she suggests it. She has always been more into me than I am into her, and this whole thing is just making me anxious and isn't nearly as fun for me as for her.

Also, in terms of the friendship side of things, sometimes I feel like she thinks (as others have thought in the past) that I'm this Byronic damaged intellectual with a cold heart and repressed psychological issues who just needs love and a girly makeover, dammit and that always frustrates me because I don't want her or anyone else to see my minor problems as any more significant than anyone else's, or feel the need to protect me. I know she means well, but it's just patronizing and weird.

I guess it's partly got to do with the gender issues I'm dealing with at the moment (which I haven't and don't want to tell her about just because I'm not ready to tell anyone rn), but for the most part, I don't know why I don't want this when logically, it's kind of a missed opportunity. I want to tell her that I want to end it, and I'm sure she'll be accepting and cool about it since she always has been, but how do I tell her and what do I say other than: 'I don't want to do this anymore because...I just don't.'?

Any advice on this would be much appreciated, and thanks a bunch in advance. (&&&)