Lessons from downhill crash
To ride in the dark is difficult.  You should have a decent head light on your helmet so it tracks your vision, and a handlebar light that tracks where your front wheel is aiming.   In addition you should have a secondary front light and two back lights for redundancy.  We don't make product recommendations here, but that would be the minimum.

To increase your awareness, consider riding UP the track that you plan on coming down later.  This way you'll be moving much slower and have a better chance of spotting obstructions before you come through.

Also consider not doing crazy speeds on downhills.  The road rules say "Don't outdrive your lights" so you should be able to stop in the distance illuminated.   Perhaps you should leave the downhill leg until the sun is higher ?

Braking technique - if you were doing a hard brake on a turn, and your front wheel slid out from under you then its all over, you're going down.  Some more tread on your front tyre may have helped, as would lower pressure in the front tube.  Another technique, you could overweight the front wheel to make the side lugs dig in and regain grip, or you could unweight the front wheel and try to stop it pushing, and therefore regain grip.   All this has to happen really quickly, we don't generally react fast enough.

Clothes - your outerwear did its job, and took all/most of the sliding damage.  I suggest you slap some patches on the damaged bits, and on the knees, and call it your Riding Gear.   No point dressing up all pretty for a ride (unless you're into that sort of thing)   My favourite riding pants have canvas on the knees, over a patch of woollen blanket, and some light cotton on the inside of the knee,  They're butt-ugly but totally comfortable.  

Finally experience - you've just acquired some.  Sadly it was painful.

You should also give your bike a close look and service.  Standard crash damage to look for includes:


Wheels out of true, rim damage, broken spokes and loosened nipples.
Disk Brake rotors no longer flat, or rim brake pads slightly out of position
Gears out of whack - bent derailleuruer hanger is very common.
Mudguards/fenders out of position, or plastic bits cracked.


Also give the bike a service and lube, and look for damage while doing it.