you have one of the least survivable specs in game, given your abilities, so you have to subsidize that with smart play, ie: don’t rush in to the fray with the group thinking you’re gonna dish out damage like a marauder or Powertech, you will just get embarrassed a die fast. But if you go after the guys with 40-60% health, you should be dropping peeps before they can counter, 90%. Even still the results will be different if that’s a Jug with low health or a Merc, so understanding your target's abilities goes without question.
if your any good as a Dec Assassin, there's no reason you should be getting owned all the time. Not only do you need to master your rotation to perfection, but you need to top out your stats, get your gear, and understand the play style. You have way less survivability than most other DPS specs so don’t play like them. Deception is not for the newb looking for the easy entry spec into PvP, go darkness or Madness if that’s your Aim.