The swordsman tree has a skill that reduces backstab damage, so exposing your back isn't as big of a problem. Other than that, try to resist the urge to roll away from enemies (i.e. back towards the camera) as this means Geralt will turn his back on the enemies. It's better to parry (which WAS changed in the EE, I think) or to roll sideways when trying to get away from enemies. Also, the swordsman tree skill that increases the roll distance is useful, as if gets Geralt out of the way faster.

I'm currently playing a mage build which means I usually open with some sign attacks and then close the distance for a few sword strikes. The Quen sign is also very useful no matter what tree you choose because it will block incoming damage, including backstabs. If you are sword-focused, you should use Quen a lot. For my mage build, I use it sometimes but other times I like to keep my Vigor available for offensive signs, especially if enemies are weak to them.

With some practice against the different enemy types, it gets a lot easier to avoid taking too much damage. Also remember to prepare and drink potions before tough fights, and to use bombs and/or traps to help out.

If you try the new Dark Mode difficulty (I am trying it for my current playthrough) you will have the opportunity to craft some really powerful equipment that lets you heal yourself when you hit an enemy with your sword. This helps a lot and gives you an edge against the extra-tough enemies on this difficulty level. But things are tough before you get it. The prologue remains very difficult, and once the game proper starts it definitely helps to learn the right tactics against certain monsters. For example, tackle Nekkers one at a time so they don't swarm you, attack wraiths ferociously, never letting up, etc. Once I learned these strategies things were manageable, though still tough, but they're definitely easier now that I've got my new armor and swords crafted.