In my same experience, I found out that it's much easier to come up with a nice theme than to write a full piece. Don't worry about fitting into a style , or whether or not it sounds like CG, there's nothing wrong with David Rock or whatever. I might prefer Santiago De Murcia, but it doesn't matter, YOU make the music YOU like.
As for your question, you asked a hard question. Getting stuck might be for any number of reasons. I don't know your playing or the level of your knowledge. Here's some things that might cause your problem, and it might be a combination of them:
not knowing much about form
not knowing much about theory
not knowing how to harmonize a melody
not knowing much about arrangement
not understanding why the balance unity vs. variety is vital
not understanding how to achieve either balance or variety
not having a decent performing technique. This is probably one of the most crippling limitations for composition.
Interestingly, I hear people who say that to be a composer you don't need to be a good player, or to know how to play in the instruments you want to write for,
but I completely disagree.
Composition, arrangement etc are the most difficult things to do. It's where everything you know (and don't know) comes together. The composer is as complete a musician as possible. Best you can do is taking it as a long term project and keep learning. If that might comfort you, even master musicians such as Beethoven could take years to finish a piece. Their compositions started to have weight only when they weren't children or kids anymore.