inspiration just happens, you can't force it.
having that said, i'm in a similar situation, meaning that i write all the music myself, except vocal melodies and lyrics, that's my singers job
so i can relate...
sometimes it's indeed hard....i feel that the less pressure you apply to yourself, the better things flow. don't overthink stuff, just write out of your gut. you can judge later if the song is actually worth ending up on a release or not, but at least finish it. perhaps you'll stumble upon an idea in the process that you can use in a different song, or that even spawns another set of new ideas that eventually turn into a much better song that you just wrote.
my best songs pretty much came out on the fly....i usually jam around until i stumble upon an idea, then build it from there. sometimes it just comes naturally, you just KNOW where you're going. sometimes it's hard work. if you're severly stuck, just leave it alone, focus on other material, and come back to it later with a fresh perspective. if you have trouble finishing a song, chances are the idea you just haven't found the proper context, or the idea is not good enough anyways. you'll know the difference when you listen back to these old riffs.
aside from that, don't limit yourself. i'm having a severly hard time trying to write in a certain, pre determined style. just let it happen.
also, +1 to what was said above. listen to different music, get out of your comfort zone, and just don't push yourself too hard. self critizism is a good thing, but overanalyzing doesn't work.
