HereForTheBeer: Well, yeah, but... I dunno. Are you playing in a run-and-gun style, or more deliberately with scouting and planning?
And use the daylights out of those binocular things to help you find and fix the mutants. Creep-mode, too, to keep your noise signature down. I remember creeping and crawling around for most of the game. ; )
I'm never running around gunning stuff. I'm crouching or crawling around in the bushes, trying to be as stealthy as possible, and using the binoculars as often as I can. Unfortunately they rarely pick up anything unless I get decent vision on enemies from higher ground, so often they don't do much. Based on advice I've read I'm supposed to be doing the right things, but it just isn't working well. Unless enemies are isolated and I can take them down with sniper or silent MP5 fire, they instantly know where I am and will sooner or later find me. Or they'll just snipe me in the head (with zero showing on the bottom-left bar) from God knows where.
Then you have the mutants that will one-hit kill me. They simply feel mighty overpowered, and I've yet to meet the really tough guys yet. The mercs are more manageable and fun (the first 5 missions were great), but it's the mutants that mostly do me. Well, the impossible sniper fire too. Enemies seem to be omniscient. I'm using the unofficial 1.42 patch too, which I think is supposed to remove some of that, but it's still pretty absurd. Had I been running around gunning into the air it is one thing, but crawling on my belly in the bushes?
Sorry to rant about this, but I really wanted to get into this game. Reading around a bit I'm far from the only one with similar opinions, though most say it's about the last third that gets ridiculously hard. I don't even want to think about how tough that must be when I'm struggling now.