When to "graduate"?

[OakLeaf]

I have to ride the dirt hills to get to the paved roads because I live in the woods. There isn't a safe place to park unless I drive for a good half hour...

I live in the woods too. I would strongly recommend that you either ride out the same way you'd take your car out, walk your bike down your lane, or drive to the end and park by your mailbox. I'm not sure how long your lane is, but however long and steep it is, it's got to be safer and less technical than what you describe! I live on a 1/2-mile crushed stone lane with a couple of steep sections, and I might do any one of the above three on my skinny-tire road bike, depending on conditions (both the condition of the lane and the condition of my head).

I pressed my brakes too hard and went head over handlebars

You squeezed your FRONT brake too hard and your rear brake not enough, and forgot to shift your weight back behind the saddle. That's exactly one of those kinds of skills Indy is suggesting that you learn. I did an endo two years ago myself, after decades of believing it wasn't possible on a road bike. That belief was what made me too lazy to learn better braking technique. I was lucky enough to escape with seven stitches in my face and two sprained wrists and thank goodness a neck that was only jammed, not broken. You can bet that I've been working on my braking technique ever since then!

I would second everything Indy said, though I know zilch about mountain biking, technique is important for everyone to learn, and as someone who's also dealing with irritated spinal nerve roots, you don't need to be putting yourself at risk.