Mountain Biking is Hard

Let's just be up front and say I don't always get the obvious..... I figured mountain biking would be just like road biking, but in the woods and sitting a little more upright. Actually, I thought it might even be a little bit easier than road biking; the big tires and plush shocks are super comfy and biking on those nice packed gravel rail trails and carriage paths with the family is pretty chill.
Imagine my shock when I went out on some of the single track trails behind my house and nearly keeled over. My first outing on the bike should have clued me in, but I figured it was just the mud and the lack of riding time.... Since then I've been out on the road bike a few times and the trails were pretty dry today.
The downhill parts of the trail were hard and terrifying. Trying to get the bike through two trees barely wide enough for the handle bars to fit through after taking a 85 degree switch back on a trail that drops off steeply on one side when you are going down a steep hill is hard. Keeping your feet on the peddles while bouncing over rocks and roots is hard.
The flat parts of the trail were marginally less scary, but more taxing. Getting whacked in the shins when your foot slips and you need to peddle because the back tire is caught on a rock hurts. Having the bike nearly stall out on you because the back tire doesn't roll over a rock and you suddenly have to peddle extra hard when you are in a cruising gear hurts. Peddling double time through a muddy patch is hard.
The uphills are hard on a road bike, but at least you can usually count on the road being straight and smooth. Navigating switchbacks without stalling is hard. Slipping in the mud is hard. Peddling over rocks and roots is hard. Keeping a steady pace when your feet keep bouncing off the peddles is hard. Not falling backwards when your front tire bounces off a rock and your back tire slides down the side of another rock is hard.
I went just over four miles on an easy/intermediate stretch of trails and it was one of the toughest bike rides of my life. According to my heart rate monitor, I was in zones 4 and five most of the time. My legs felt like goo, and were bleeding from a few spots where I got hit really hard with the peddles.
Don't get me wrong, it was a blast and I plan on keeping it up. In fact, aside from the pain and need for an immediate nap, it was one of the funnest and enjoyable rides I've had in a while. The thrills were nicely balanced with the peace of being in the woods.
I think I may need to get a little professional schooling, or at least a lot more practice, before I move onto any more intermediate trails.