In reply to Richard Carter: FWIW I stacked at well over 35 mph after a horrendous speed wobble last year, was descending quickly and with confidence up to that point and totally lost my descending mojo. All winter I was a gibbering wreck and could bring on a speed wobble at 15 mph just by being tense which really didn't help with the fear! But I persisted and have regained my confidence and can descend well over 40 nice and relaxed again. Being relaxed in your mind and on the bike is absolutely key, all the techniques in the world aren't going to work unless you relax.
What I learnt through that process may be of some help to you. I don't like big open hills or long straight descents of the same gradient (as this is where I crashed) I prefer technical descents which give me much more to occupy my mind than a straight descent does.
However some hills somehow just feel safer and I searched these out and practiced my descending on those and held back on the bigger ones (I didn't really go looking for them but where they were on regular training loops I just tried to descend as best I could).
I quickly found that on the ones I felt safe on that I could easily get over 40 by not braking, carrying the momentum through, tucking in on the drops etc. As I was relaxed all these things were much easier and then descending at speed became a lot of fun again and I would relish doing it on hills I felt safe on. So on the same tame hill I could increase the mph by a few more mph after a few goes just by relaxing and doing all the right things.
As I became more accustomed to doing that I could then transfer that to the hills I didn't like until I felt completely relaxed on all the hills (except in strong cross winds still hate that!).
What I do is tuck in once I start to spin out at around 36-37mph on a 52-12, get in the drops and tuck in and concentrate on having relaxed shoulders and arms and to grip the bars loosely and allow the bike to 'float' under me. Brake before corners and mostly with the front but try to brake less and less if I can to carry the momentum. But being relaxed is the real key, being tense makes it more likely you'll crash!
I also helped myself with this by buying a new bike that descends like a dream! It's so confidence inspiring but I realise that's not an option for everyone, but it worked for me. Good luck!