Indigo, I think about the libido thing, what I really want to know is why it's a problem to you.
Is it because of the disconnect between your libido and TP's? Or is it something else?
Having a low libido, in itself, isn't necessarily a problem. It doesn't mean anything about you "not being able to want her" any more than her higher libido makes her sex-crazed.
What often causes problems is the difference between two libidos. And I've known that, and I felt terrible. Just like you I was the partner with a lower libido, and it became lower and lower with time. And I felt terrible that I couldn't provide my partner with what's considered so essential in relationship these days (regular sex). Especially when he didn't have other partners.
In my case, it became worse and worse because I felt worse and worse. My libido is a bit higher now that I've stopped feeling guilty about it, but it's never going to be up the roof. I need to be fine with that.
It's hard to know what's causing you to feel bad. Is it yourself? Is it other people? Is it the comparison to TP, or to other males? Either way, I think I would work on that most of all.
I found that sex became less scary and less daunting when I wasn't expected to provide it. Then it could become something I wanted and enjoyed, and not my duty and responsibility and measure of worth. Most of all, it became much easier when I realised I was only responsible for MY sexual satisfaction, not my partner's.
TP can take care of hers. She has other partners, and I'm sure she knows how to masturbate, too. And with you she can get cuddles as a form of intimacy, I'm sure (unless you are having a problem with these? I'm asking because at some point, I ended up having a problem with even these, because I felt that my partner was resenting me when they didn't lead to sex. So I avoided them altogether, too).
You need to try and let go of your feelings of inadequacy: you're responsible of your own happiness, of your own sexual gratification, of your own everything. Not of hers. You're here to help and support her, but it shouldn't be your burden to match her libido. I'm sure she understands that, but you need to understand it too, not just mentally, but from within. That's own I believe you can get better.
I've been in both positions (of having the higher or the lower libido) and I think that perspective really helped me: when I wanted to have sex and my partner didn't, I realised it really wasn't a big deal at all. That made me relax a lot about the other way around. It always seemed much worse when I was saying "no", like I was denying them some kind of essential right.
But it's really not a big deal. I much prefer being the person being said "no" to than the other way around. Then you just need to know not to insist, and ask if something else (cuddling for instance) would be fine. And sometimes I enjoy the cuddling more than I think I would have enjoyed the sex! As I said, it really helped me not feel guilty (although I'm not completely better about it, at least I get over it much faster).