Best way to learn jazz theory is by playing it. Lots. With lots of other jazz musicians.

 

Theory will NOT teach you how to compose.

 

That is something you feel by doing lots of practice combined with a little trial and error, a little plagiarism, and a lot of deciding on stuff that you really like.

 

 

I am not trying to tell you not to learn theory - on the contrary, it is a great idea, and essential to extending yourself as a musician - but it sounds like you haven't really "found yourself" as a musician yet, if you find composition difficult. It should be second nature - and can be, in my experience, simply by doing lots of it, and being prepared for a whole bunch of turkeys before you write something you like.

 

Often, you'll find that revisiting old material suddenly gives you fresh inspiration, as you hear something in your music that you definitely didn't notice while you were writing it.

 

Of course, if you don't write loads of stuff, then this won't happen... /ends nag

 

The point I made earlier about plagiarism is more serious than it comes across; one of the very best ways to improve as both performer and composer is to play loads of other people's stuff, and pinch bits that you really like, and use them in your own compositions (the ones that you're "never going to let anyone else hear"), or just play the songs your own way - add a twist. Maybe play a familiar song, but completely re-write the bass line to suit your style.

 

All composers of any note have done this sort of thing, and, as you learn nifty tricks and licks from other people, you play with them, improvise around them and hey presto, you've written something new, and you feel a bit better about yourself as a composer.

 

This is the bit I'd look at before diving into more theory books - make your existing theory knowledge come alive.

 

Just a suggestion.