I can hear a tune .. hum a tune.. come up with a theme in my head.. but for the life of me, making that tune come to life is just beyond me.
Don't be discouraged. It takes a
lot of time and practice to get music to sound like it does in your head. I've been composing and recording music since my arms were long enough to reach a keyboard, and I still sometimes have to settle for results that don't quite match what I had in mind.
I kind of say this to everyone, but I honestly believe it: whether it's playing an instrument, composing, mixing, mastering, or whatever else, the only way to get better is to keep at it, even if it takes years to get to a level you feel satisfied with (heck, I know I'm not satisfied yet). The more you do, and the more you experiment, the more little tricks and secrets you'll figure out to make it better.
Something I'd recommend, if you haven't tried it already, is grab some of your favourite vidjugame music, and try to recreate it, as accurately as you can. Doing this back in my MIDI file days really helped me to get a better grasp of what made all the elements work together, and I still do it occasionally nowadays, just for practice and fun.
When you feel like your compositions are solid but they lack the polish you'd like them to have, try getting into production magazines like MusicTech and Sound On Sound - they mostly tend to review gear that's coming out (and oh goodness is it fun to drool over), but they also do a lot of tutorial articles that are packed with good tips and advice - a lot of what I learned about mixing and mastering in college, I came to realise, could also be found in these magazines that I was collecting at the time, to a degree. If you don't want to wind up with stacks of magazines, you can buy PDF versions online. YouTube is also a great source of such tutorials, and also has the benefit of providing audio examples on-the-fly.
Good luck, and don't get lost down that rabbit hole. ;P