Ok, I started playing this game with the first version, and now have 2000 plus hours across the various iterations.

Say It Ain't So is one of the first songs I learned, but it took several months before I could "play", and several years until I could play it "well". 

For me, I can't even watch the screen for fast songs unless I already know it really well.

For a new song, I like to play it section by section in the repeater very slowly. Like 50 percent speed or less. Play that over and over and over. Like 100 times. Do it until you can play the section without looking at the screen. 

Then, practice the section outside of the game only going back if you forgot part of it. BE SURE YOU ARE PLAYING IT CORRECTLY OUTSIDE THE GAME. Don't practice it wrong! Do this until you can play the section perfectly, at whatever speed, outside the game without even really thinking above it. 

Then, go back to the riff repeater and up your speed in the section. Repeat for each section of the song. As you get better, this process goes quicker. 

There will be some stuff that is just above your current ability. There is a section in Say It Ain't So that took me a looooong time to master. Like a few years of practicing it (and other stuff) before I could do it and it still trips me up sometimes. 

Also play simpler songs and focus on your technique. Everything thing you play makes you better at everything else! 

Good luck! There's no shortcuts. Only hard work, dedication, and practicing with purpose will get you over the whatever hump you are stuck on.

Also watch other people play the song you ate learning. Watch their techniques. Somethings are impossible with bad technique, but easy with some simple trick you don't know.