You have willingly signed a Legal document I doubt a good lawyer would be able to prove duress was applied to make you sign something.
The only way out is if they no longer employee you of their choice. My training bonds were "bought" by my new employer and guess what.. I had to sign a bond for that.
Its unfortunate when other factors implied or promised do not happen whether due to unforeseen circumstance or down right decit .I've had both but was fortunate in that I found a new employer to take on the bond in 1 case .In the other case i waited for the bond to expire and left as soon as possible.