Why won't iTunes sync to my iPhone?

When I plug in my 4S to my iTunes library, it naturally does its automatic sync. However, when I click on "On This iPhone" and my music library is pulled up, the music that I see on the screen is NOT on the phone itself. For example, I bought and synced Justin Timberlake and Eminem. These two CDs were imported to my iTunes library normally. When the phone synced the audio, and told me that the sync was successful, the music never showed up on my phone. Even though it SAYS that the music is "On This iPhone".IT ISN'T. I'm more than frustrated and I don't understand what more I could possibly do to fix this problem. That's my main issue. Furthermore, music I have bought over the past years on iPods and whatnot that stayed on my device shows up neither in my music library or in my phone's library when it's plugged in. These are two issues I'm in desperate need of fixing, as no one I know has the same issues. I don't want to call Apple Support because every single time I have attempted that in the past, they were of absolutely no help. My iTunes has been a mess since my early years of high school. Someone help me please!

Take it to an apple store. They will help