Mismatched libidos are common it seems. I wouldn't like it either. 

You say she likes it when you do have sex, though - could this planning actually be kinda detrimental? Would you have more luck if you focused on 'seducing' her/turning her on, rather than planning it and then just quitting on the idea when she inevitably doesn't feel like it? 

If she isn't reliably feeling horny, then arranging a sex session might mean it's playing on her mind and she's actually dreading it come the time, further putting her off sex because it's making it an awful thing. My libido is really high to begin with, higher than my partners, but I think I would be put off if I were told 'We are gonna have sex tomorrow night!' too. I dunno if I am gonna be in the mood then, it makes it feel like a chore, just no.

If my partner *does* want to have sex when I'm not in the mood, he can make me go from 0-100 in the spur of the moment with certain things (for me, massage that gets progressively naughtier, playing with my nipples, etc). But yeah I'm easily turned on in general, but you might find something that works for your partner.

https://lifehacker.com/how-to-identify-your-partners-libido-type-and-get-bet-1692431142

Give it a read and see if you can identify her and whether you can look up anything that helps you.