Just put your phone without battery in a ziplock bag in the freezer for 1 hour (not kidding). Let it sit for another 1/2 hour at room temperature, then insert battery and try booting into fastboot. If it doesn't work, remove battery and try again in half hour. Try until it boots, I tried about 15 times before I managed to boot into fastboot mode. So don't give up.
But first, read my guide on how to really go back to HTC stock ROM and solve boot problems - noob guide, you must have everything ready when you manage to boot your phone: