I can read music, but only if I play the score with my guitar. Since this will undoubtedly make composing harder, I want to be able to hear sheet music in my head without the aid of my guitar.
What are some good tools/resources/techniques for learning how to do this?
The best way to do this, is to practice transcription, and sight-singing. The goal, ultimately, is to be able to look at music, and hear it in your head. This takes time and practice, but you can learn it. Pick a simple piece you know well, like Happy Birthday, and then attempt to write it down by imagining the notes in your head. Once you’ve written it down, play it on the guitar, and make corrections, see where you went wrong. Do this with other pieces as well.
For sight singing, just pick lead sheets, and attempt to sing the melody. When you get stuck, play the current note on your guitar, and then keep singing after that.
Practice is really the only way.