Same to me, bud. Sometimes I get a freakout from this game, but obviously, the removal of autoaim with controller is what makes you "bad". Every console game has autoaim in it: CoD, Halo and Crysis 2 of course, but in the PC Version it got removed. I really helps you if you try to use keyboard and mouse. My aim was like garbage at the beginning, but the longer you play, the better you aim.
Then there's the movement. Don't always run around in Cloak. It does not only drain your energy too much, it also weakens you totally. Only turn it on in critical situations, or when you have to switch positions. PC users sit in front of the monitor, so they see cloaked persons way more often. Don't rely on cloak.
Look for movement trails to run around the map faster, Skyline has multiple ones. I found about 5 or 6 shortcuts, when trying to get somewhere, and appearently I'm the only one using them.

So don't use the stairs, jump onto it!
If all this doesn't help you, try to use Pointfire Enhancement with a Feline or SCARAB (if you got them) and pwn them all away, it is really a good combination. Just don't use ADS when trying this out.
And ALWAYS, really ALWAYS switch to Armor mode before starting the fire. That sets your survivabilty up about 50%.