This might be because you are either using regular swing with one dagger or power attack with one dagger. You really should use, at all tmes, either Dual Attack with both daggers (both mouse buttons) or Dual Power Attack with Daggers (Hold both mouse keys). As soon as you start playing like that, daggers start doing a ton more damage. Also, it just takes a bit practice to lure multiple enemies out. For example, If you have multiple guys that are standing together, just shoot your arrow once, on the ground preferably because hitting the enemy opens up the sneak eye more then not hitting them, then let them come towards your direction to search. Before they reach you the eye should be closed. So while you re-gain hidden status, start approaching the last guy from behind and take him out. Rinse and repeat.
If that doesn't work and you get detected, just sprint towards opposite direction and hide until you are hidden again. Low level play as a dagger/dagger assassin has to play like this to be viable until you get higher. Or alternatively, just use your bow more.
As above, its a rare thing that enemy stays alive if you execute back-stab properly, sometimes enemy has like 5% of life left which isn't a problem, use two buttons simultaneously is a must.
Also shooting arrows in the walls to make noise and lure enemy helps a lot. I have no problem killing stuff and I'm only lvl 21.
Dragons need to be ranged though...