That happened me too.
You have to start a new game, but don´t worry, the missions look easier, I´ve reached that mission like 1 week since i started the game.
Another thing: do not activate again the riot cheat!
I love ¨The Sicilian Gambit¨.
That´s the best mission in the whole game, I really enjoyed the game, but I want to buy the Manhunt 2 for the PSP now.
Nice work Rockstar!!.