Rob: Let me just touch on a few points...
1. Footwear. If you are in fact gettting within 20 yards then you have the method for walking quietly down pretty well. My only question is your footwear. If you are wearing regular hunting boots with lug Vibram soles then you may as well have bells on your ankles. Vibram lug soles are the WORST thing you can wear for this kind of hunting. If it's dry and not frigid cold I wear Converse high top sneakers for my hunting. I buy them a size larger so I can wear a couple pair of wool socks. I also have a pair of Tingley slip on shoe rubbers for the early morning dew. When it get's colder/wetter I then use LL Bean Maine hunting boots with the thinsulate liner. You need footwear that you can "feel" the ground with when you set your foot down before you set all your weight on it so that if you feel a stick you can avoid it before you step all the way on it.
2. Binos are a must, especially in heavy cover. You HAVE TO see them first or the gig is over. I know that's tough but that's what makes it so much fun and what is so rewarding. When you see them and they don't see you with this kind of hunting, especially when you're close, then you've won, whether you shoot one or not. I use mine constantly, and will scan a cover up close, and then refocus so that I can see "into" the cover layer by layer. Obviously you look for movement...ANY kind of movement. That's a no brainer. But you also need to learn to look for horizontal lines in a setting where everything is vertical.
3. Patience. That's the key. I can't speak to your PTSD but for me I couldn't hunt this way until I had developed the patience to sit from dark to dark in a treestand. Hunting this way takes a LOT of self control and discipline. I find that I need to convnce myself to ABSOLUTELY believe that there's ALWAYS deer just beyond where I cannot see and I need to move accordingly. In fact, another mindset I use is after I've scanned a cover and am 1000% positive that there are no deer IN THE FIELD OF VIEW I JUST SCANNED, I then hold the advantage. When I move then I GIVE AWAY that advantage, so when I do move I want to do it with extreme caution...like walking on eggs. Then I take a few S L O W steps and the stop and do it all over again with that mindset.
Yes, this is slow hunting. If you are doing it right the cover you worked though an hour ago..which may be 40 yards or so behind you...is just as viable a place to spot deer as what's in front of you so you have to scan behind you just as much as what's in front. This takes A LOT of patience like stand hunting, but it differs from stand hunting in that it IS NOT boring. You always have something to do. In fact, for me I find that sometimes I just find a log and sit for a while because it's so intense that I need a break! Go light. Leave the cell phone in your truck. Forget about time also, or at least in the context of the hunt and the idea that "I want to get to the big oak near the pasture by 10, so I can leave by 11". If you are hunting according to a watch then you are beat before you start. The ONLY goal you should have is to get another few yards without being seen....period.
I hope this helps. Best of luck to you.