Here are some ideas for the aspiring Composer-Guitarist to try. As with anything, use what you can to fit your own personal needs, and disregard the rest.
a) Teach yourself how to really listen to music, to all the different voices in a composition. There are typically more layers in classic Art Music (Bach, Haydn, Mozart, Beethoven, Wagner, Mahler) than in Metal/Rock. So really expose yourself to that as well as the heavy. There are some fascinating things going on in real Jazz music as well, daring contrapuntal stuff. You can also learn a lot of invaluable things about tempo manipulation and polyrhythms from funk, reggae, and rap. Actually, that's kind of a bigoted statement, since you can learn more about those musical variables from all kinds of music. It's just that the above mentioned genres can give you ideas from outside the metal/rock/classical box. Don't blindly write off any type of music; limiting your musical input ultimately cheats you from both educational and entertainment perspectives, and can lead to bigotry in other areas of your life. Just as you can learn something from any person, you can learn from any type of music...even if it's just to verify that you don't like that particular piece of music (laughing).
b) While you're working on building your listening skills in music, strive also to prune your listening skills when it comes to the opinions of others. You are not obligated to take to heart the opinions of anyone else, and you are completely free to take whatever is positive in any situation and turn it into a self-affirming experience.
A warning: the smallest people will try to convince you that their reality is all there is, when that's all they're really talking about, their reality. They will try to infect you with their smallness because it's the only thing that makes them feel bigger.
c) I urge the composer-guitarist of today to work toward being the best, the trailblazer, the Landmark in his or her genre. Don't let yourself stay overawed, and don't let anyone make you afraid. You have to make yourself what's happening. Your day is here.