Oooof....A lot to answer there.
Are you recording your tunes?
One thing to experiment with is layers. Especially with slower and/or simple riffs, layering can be a big key to bringing a song to life. Think in terms of an orchestra...if you just sat & listened to just an Oboe play it's part of a huge arrangement you'd be thinking wah?!?!? But put it in context with all the other instruments around it and it makes sense.
A lot of times guitarists...especially Rock/Metal guitarists...get caught up in thinking that THE RIFF is THE ONLY thing that's important. A guitar's range covers a lot of sonic territory, so why should we have to settle for just chugging away in the low registers for rhythm and soloing up high on the neck. Mix it up and layer it up.
A dude called the Ulnarian over at Guitar War/Musician War explained it the best....he used the choir comparison....you listen to one member of the choir & they suck, add in 50 more so-so singers and you'ge got a choir...and it sounds great. LOL

Now I'm not at all saying that you suck (LOL)....just that adding in layers can lend substantial interest to a section that might otherwise get too repetitive sounding. Write a simple, heavy riff and just start messing with additional riffs/lines that play off of it and compliment it. Then, the funniest thing is, you can take away all those layers and go back to just the underlying riff and it will sound almost new again. Eventually, you'll learn to strike a balance between using heavily layered and sparse textures as you build your style.
Of course beyond that even, tones & effects can also bring things together. Using bright, clean tones to offset the heavy dark ones...etc...etc.

Hope this was somewhat helpful...cause I type very slow & this took forever! LOL ;)

best of luck,
Don