The specific interval website helps me so much. I like how it tells you how many half steps are in between each interval and then you hear audio of the interval and also you see it written in notation. THANK YOU!
Many people will tell to do this, or do that, but the one thing that your missing is relaxation. So if you say you can't get the sounds from your head onto a guitar, then put the guitar down and go watch a movie (Relax). Then when your no longer tense sit down take a deep breath and try again. Doing that has helped me for about two years and i managed to create hundreds of riffs and songs!!!!!!
Then your saying that you to created many riffs but they sounded messy.
I think the best thing would be to meet some people that play the music as you.Hey you could even form a band. Cause when you play with people the same or even greater skill then you. You then tend to learn from them. :)
