Can't transfer music or videos from itunes account to iPhone 4

PLEASE HELP!!!

Here's my situation. My computer completely died - unrepairable. I purchased a new one and went through the process of reinstalling my itunes account along with songs, apps and all that jazz onto the new computer through my itunes store purchases. I authorized my computer to allow my iphone to play music, backup, sync and transfer purchases from my iphone to my computer but I can NOT transfer ANY music from my computer to my iphone (whether it's from a cd I added to my itunes account, music I purchased from my computer online or the itunes store...). I figured if I could set my iphone to manual sync that would fix the problem but I am unable to do that without getting the erase and sync message. Since I am not able to do that, I set my options to automatic sync all checked songs and videos and still nothing. I have songs on my iphone that was from my trashed computer that is not on my new computer that I was not able to back up on an external hard drive and unable to add to my itunes library for the reason I do not want to erase and sync. I don't know what to do and it's driving me insane. Please advise. Thank you!