First of all, there is never a need to ghost a partner or a person, no matter who incompatible they are with you as a friend or roleplay partner. If you don't feel confident enough to come out and say why you're not interested in roleplaying with them, I'd just politely tell them that the roleplay isn't what you're looking for at this time and, that you're sorry, but you don't wish to continue the roleplay. It's polite to at least tell the person you're leaving the story, even if you don't give specifics why. That way, they aren't left hanging forever.
So, that aside, let's move on.
When it comes to roleplay, I've found that I don't have time for roleplays I don't enjoy. I'm a busy person and roleplay is a hobby for me, not my job. Perhaps I'm blunt, but I'm not afraid to drop someone if I'm not enjoying a story. Telling her no isn't ditching. Ditching is saying yes, starting, and never responding again.
In this instance, it sounds to me like you're yanking this girl's chain a little by kind of committing, but not really committing to roleplay with her. Though it may not seem like it on the surface, it sounds like you're being ruder to her this way (this non-committal, I want to be nice but I don't like you sort of thing) than if you just told her you weren't interested in writing with her. You don't' have to be a jerk when dropping someone, but dragging them along and keeping their hopes up is the worst thing you can do to a person.
If you're still feeling hesitant about that, have you considering politely asking her if she's interested in getting some lessons? Even if you don't want to do it, personally, there are tons of writing/roleplaying coaches floating around. I bet several people on this site would be happy to help her, but asking her if you she wants you to just write her character isn't helping anyone. It's a waste of your time, it will probably crush her feelings, and everything will be worse off.
I'm going to be honest, but it sounds like maybe you're not the person she needs to guide her and help her develop her writing. That's not a call on your character or skills, but if you can't put up with someone needing a heavy hand in guidance, offer to help her find someone who will. (: Just my two cents. Good luck in your situation!