synchro

Hey guys, I really need some help! I have a 2005 Mustang GT, manual transmission with about 146k miles on it. The problem is that my 2nd-3rd gear shift almost always grinds, even if the clutch is fully depressed. I understand this is a synchro problem, but the real question is, I never abused...

So The synchro gear in my 3rd gear in my 2001 Bullitt, it is gone, i need advice on the best way to go about fixing this please.