TS1503 URGENT PLEASE HELP

I had to recently restore my computer, so i put all of my music on to a flash-drive, and i put it back on to Itunes. I want to sync the library to my phone again but it's acting like it wants to add this new library to my current library, and saying i don't have enough space on my iphone for it to sync. It's acting like it wants to add this new library to my current one, which is pretty much the same library. PLEASE HELP, i need sync my Iphone, and the only option i can think of is deleting each one by hand, but thats 700 songs! There has to be some way to erase my old library off of my phone and sync new one, but it's not giving me any options at all!, and if i restore and backup iphone, it would just sync back the library thats already on my phone changing nothing..