What helps me is focussing on form, and doing all the technical things correctly. I go through a checklist, basically: Put all your weight on the outside foot. Keep your centre of gravity low (in the drops if you can). Look ahead, pick a good line and follow it smoothly. Stay of the brakes*. And, most of all, I tell myself to steer the bike with my butt and not my hands. (Which also includes not sitting firmly in the saddle, but I wouldn’t do that anyway when descending).
*Regarding braking. Both cornering and braking reduce grip, so don’t do both at once. Brake down to a safe speed on a straight before the corner, then let go. If it’s very steep, and you’re picking up too much speed through the corner without braking, drag the back brake slightly. Stay off the front brake.
First would be to practice on some of the less technical or less steep descents. You’ll never improve (and likely get worse) if you only practice on descents that put you too far outside your comfort zone.
Second, on those less technical descents, you need to get off your brakes. I know that sounds like I’m just saying “Descend faster!” to your problem of slow descending. But the reality is that your bike will handle much much worse when you are riding the brakes. And this will lead you to feeling much less confident and able to control your bike.
Third, session. Find one of these less technical descents and do it over and over again. That way you know there’s no pothole around the turn, you know it won’t flick back the other direction and mess you up, you know there’s not a stop sign right on the other side. Just ride up and down the same hill trying to have less brakes, more speed, more confidence each time.
Finally, all the normal advice of eyes up, look where you want to go, weight on outside foot and inside hand.
I live in a very flat area (Chicago) but just got back from my annual trip to N. Michigan where the roads go sharply, constantly up and down in the 15%-18% grade range. I consider myself a confident descender, but the first few days I ride up there I always feel squirrelly going downhill … especially when I reach 40+mph … but as I start to diagnose my jitters, I realize that as soon as I fix on a point WAY up the road I become much more calm. It takes me a day or two to remember this fundamental, because I really don’t have a need to practice it the rest of the year.
I think our brains get over-stimulated when we can “see the speed” by looking too close off our front wheels, but if you fix on a point in the horizon it quickly reduces/eliminates that agitation … it’s like your brain becomes over stimulated and goes into a fight/flight mode.
There is also a “cheater” technique you might want to try until you become more comfortable: as you go down a hill and raise your weight off your saddle and rear-wards while making sure you have a light touch on the handlebars … you can pinch the top tube with your knees. This will give you an overall much better sensation of stability and will make it a bit easier to keep your weight where you want it which will make you feel more confident.
Good luck🤘
EDIT: Others have said “try” and get in the drops and “try” to raise off the saddle … to me this is 100% necessary to go downhill, much less be good at going downhill. You NEED to be in the drops … if you’re descending on the hoods this quick change will change your life. And, you NEED to have no weight going through that saddle. Think about trying to get to as close to 100% of your weight going through that bottom bracket … literally as much as you can. If you think about centering all of your weight there is will keep your hands light, your center of gravity low and your weight back where it should be. You’ll never get 100% of your weight through the BB, but if you try, a lot of other things fall into place.
Finally, I would practice descending and cornering as separate disciplines. Pick a straight hill and session it and get used to the speed and control. Then pick a flat road and ride fast into a corner and work on that skill … get good a both of them separately. Then put it together.
*As others have said, Mountain-biking requires all of these skills all the time without pause, so if you can do that it forces compliance. Do it with a dropper post and life will suddenly make sense. And it’s fun.
I’ve seen cat 1 team mates and pros on TV make these two mistakes.
Bad descenders will turn their shoulders in the direction of the turn. Doing this takes weight off the outside foot and can twist your hips which will move your CG to the inside (off the bike) and not over the bike.
Bad descenders seem to take a super late/low line. They always seem to turn more after the apex. Use more of the lane if possible and go outside to inside to outside hitting the apex on the inside. I don’t know how to describe it eloquently but, try and take a higher line or complete more of the turn prior to the apex.