If only I could play what I can sing (or think).
If you really want to then it's quite doable as long as you're prepared to put in the time practicing it. Play a note, sing a phrase starting on that note and then try and match it on the guitar. It can seem really difficult at first, but if you:
a) Make sure you start with really basic things (small intervals, all notes from the same scale) and gradually increase the difficulty
b) Keep on trying even if it seems you're not getting anywhere initially
you should find that make good progress.
Singing along to scales/exercises/etc. as you practice them will also help with this as you're reinforcing the connection between what you're playing and what you're hearing - eventually your fingers will naturally start to go to the next note you hear.