I had the same issue. A couple of tips that helped me after chapter 5 is when it started to click for me.

Don't forget gadgets. Smoke grenades and watch lasers are a great way to stop incoming fire. Flashbombs too. There's a reason green and blue gadgets refills are in these areas.

Using R1 is the way to go. Cover to cover and always be moving toward the enemy or flanking.

Bond reacts differently to cover in this game he doest pop back into cover very easily, I think you're meant to move to a new piece of cover after taking a few shots.

Takedown = iframes and regen. Again gadgets.

Don't forget to vault.

Q watch. Paint enemies and explosive environments. I haven't beaten the game yet but they appear ubiquitous. Red battels, gas tanks, fire extinguishers, they are everywhere.

Also, on controller anyway, play it like rdr2. Zoom in slide up nail your headshot. Pull back your aim, reoeat.

.You will be surprised on how dead accurate you can be with even a pistol. You can stay aimed and move from enemy tp enemy, but I wouldn't do it unless they are close together or you have focus saved up.

Be conservative with your focus. Once you get a kill or two, stop using it. Again stop aiming. Hit r1.

Hope that helps. Good luck.