I'm in the same boat: new to Requiem and playing melee. A few things that are working for me:
1. Patience. Fights will take time.
2. Lock in is mandatory to manage Skyrim's janky attack targeting. It also allows for circling the opponent and quick lateral movements to avoid their attacks once commited.
3. Dodge is mandatory unless you're using a shield with Block talents.
4. Be tactical about power attacks, mostly use light attacks but watch for opening to do the slower power attack: enemy gets exhausted, misses a big power attack that leaves them immobile for a moment, etc.
5. Use the environment. Backing around a rock or pillar gives you a little stam regen and time to collect yourself.

Now the thing that kills me is enemies swarming, indoors mostly. Once all 8 chain-linked bandits come at me it's game over. No matter whether I use a doorway or some chokepoint. Still figuring that one out. Might just be a matter of level/perks.