Great read, lots of good info. Good to see others have progressed through their dedication.
I'm in the same boat as many here, picking up trail riding late, after many years of riding on the road. I've gone from getting a good deal from a neighbor on a Honda CRF450R "setup for the woods", to a KTM 450, and now Phil's old EC300. I'm by no means fast, but mid pack in our groups which include all different skill levels usually.
I wanted to add a little about mental approach, since I feel it is a big part of being successful when riding more advanced terrain. Being prepared for the mental challenge of line selection is key, as one of my buddies likes to say "this ain't piano lessons". It is in your best interest to be sharp and on point when getting into tight woods.
My approach started at:
1) be tentative about every obstacle (normal when starting out)
2) hope there wasn't something ahead I couldn't get past forcing me to turn around.
Has now progressed into:
1) I'm not going to stop until the next road intersection or I find the wheel in front of me
2) How can I use those SAME obstacles in my favor to HELP me through the trail
3) You're not there until you're there, too many times the last, easiest root or rock reaches out to bite you because you let your guard down
Just having a different mentality has helped me quite a bit. Some of my buddies think it's the bike, but that's probably because they still ride MX bikes.
Also I found that a book from one of my old baseball coaches has come in very handy.
Sports Psyching: Playing your best game all of the time by Thomas Tutko