The ability to hear inside your head what's going on without the aid of an instrument, or the computer is something that can be gained little by little with effort and experience...
That said, the piano/instrument/computer are all tools that help you hear what you're composing. They are not there to compose instead of you! If you have problems composing this is not because of the tools you're using but because of how your mind works right now.
You need to build some confidence. Try something out and then post it here (for example). People will offer valuable feedback and you can take what you feel is worth taking... Then try again, and again. You can't fly before you learn how to run and you can't run before you learn how to walk, and you can't walk before you learn how to crawl and you can't really crawl if you don't know how to breathe...