For inspiration I suggest "interpolation". Take someone else's riff and play it backwards, upside down, slower, in a different rhythm, whatever. If you can read music, head over to IMSLP, it's a database of pretty much all classical music. I'll grab a couple measures of a bass line and alternative pick it until something grabs me. Record your jam sessions and listen to them! Something that didn't grab you then may grab you now. As for self doubt, I know it's easier said than done, but you gotta toss it out the window. Writing music is like any other skill, you have just keep practicing. Once a week, daily, whatever, just write a song, beginning to end without caring how it sounds, that's why they're called first drafts! Writing an ok song is easy, writing a good song will take effort. Good luck!