The problem is that you're a thief/rogue type of character and you're trying to fight toe-to-toe. Unless you're decked out in heavy mail, I suggest you play more to your type's strengths, and make maximum use of the sneak attack crits.
Unless you also level up your armor and weapon skills, going toe-to-toe with some enemies is just simply a bad idea. You have to play according to your character's strengths if you really want to get the best out of them. Stealthy types are more about maximum damage with sneak attacks than about just getting in the enemy's face and swinging away with a heavy war axe.
And yeah, if you're going to emphasize stealth, forget having companions, unless you just let them draw all the heat while you stay back, hidden, and sneak attack with arrows or a dagger. Just be sure to have some healing magic for the companion, if you don't want them to die at some point.
Another tip as a stealth player, if the enemy are coming in your direction, use cheap arrows i.e iron, and fire them against a wall in the opposite direction form you, and they will get distracted easily. Also a good way to lure them into the open, if you have found a nice dark spot to hide in.
This, as well. I found this out, too, and it's become an excellent way of controlling mobs and setting up attacks. They will always follow towards the sound of where the arrow struck. Hit them once for the sneak attack, and then immediately fire one shot over their heads or at the ground near their feet, and they'll keep turning back to that instead of following the source of the arrow's direction, which would be you.
Like I said, play to your character's strengths, because without good armor, you're not exactly going to be able to emphasize stealth and just whack away at everything toe-to-toe.
It's the same principle for someone who wants to be the knight with a big sword, and instead uses magic while leveling all the perks related to armor and one-handed/two-handed and nothing in magic. It'll take forever for you to kill anything because you'd be playing against your character's strengths in that scenario.