Your title isn't entirely true. I suck hard at Halo Multiplayer, I play other FPS games like Battlefield and others similar to it. It took me four hours spread over two days to get Thorn. What the real problem could be is your state of mind when you play or your in game strategies. Maybe you keep playing in lobbies with players who have more skill or you could be unfortunate when it comes to this particular weapon. Whatever the case if you really want Thorn then keep trying. Of course you'll need to earn the bounty again, but just go in there with a positive attitude and some good teammates. You'll get it just keep trying.