I think it's a really common problem amoung artists, getting their creativity from the form of idea to some sort of medium.
The best way to combat this I think may be to do what you're already doing; practice your craft.
Learn as much as you possibly can about music, writing and visual art in order to be able to express yourself in them more freely. It sounds a bit stiff but the more you know about the technical the easier time you might have being able to get your creativity across in a physical form.
Also, it's a lot about relaxation though. At least for myself when I can forget about so much pleasing others and even me and just engage in art for art's sake I can produce things that are often much more beautiful.
Playing an instrument loosely or writing and painting, sketching, sculpting, etc. with a great flow can get tons of ideas out then you can always revisit them with greater scrutiny and mould them into something more than they were before.
It's just like jamming with another person/people or on your own can result in some fantastic songs coming out. Accidently getting a terrific riff out of just experimenting then going back and refining that, adding to it, structuring it and adding to it can lead to a whole song and the same goes for sketching out vague ideas or free flow writing where you forget about being cohesive and just let go with anything at all that comes to mind.
And about the girl - I'd just take the course of playing it by ear. Keep hanging out with her as a friend if you enjoy it as that and just see what happens and go with it if you're willing to.
Hope any of that helps.