Having trouble writing music?

I've been playing guitar for over 4 years now, learned by myself, but I haven't written a 'good' song yet! I'm 16 years old btw. If you'd listen to Keith Merrow (search him on youtube), Nevermore, Slayer etc. you'd have a pretty good idea of the music I want to write. But when it get's to complicated riffing like with Keith Merrow and Jeff Loomis/Nevermore, I just can't get the job done! I can write a Slayer type riff kinda easily, but when it gets to the more complicated stuff, like I said, I can't get it done. Is it maybe because I don't know the fretboard well maybe? I mean, without trying to be a ego freak or anything, I am incredibly skilled, and a VERY fast learner compared to everyone I know who plays guitar. For example, my brother has been playing for 9 years, and after playing for 2 years myself I was already better than him, and he is Good! But as I was saying, even though I can play fast and all that, I don't know any music theory, I only know the pentatonic, blues, diminished and harmonic minor scales. And I also don't know many chords and I have no idea what major and minor is, I don't know the keys, and if I manage to write a riff and want to write a solo or chorus over it or something, I don't know where to start, so I have to slowly play up the neck until I find the note that fits into it, and try to work from that. A bunch of stuff like that. Basically, I don't know the fretboard, and I don't know how to learn by myself. Any pointers, or anything at all, that you can say that could help me get better at writing?