Yeah, that's one advantage of being on a fixie: you just can't descend that fast. My comfortable limit is about 56 kmh (35 mph), but I usually don't go above 48 kmh (30 mph). A lot of scrubbng speed with the brakes, so the pads wear out quicker, especially in the winter when it's raining. And then, when it's snowing, I'm sometimes sliding down the hill with the rear wheel locked up at about 10 kmh.
So I can imagine hitting a deep pothole at speed with the front brake on, ouch. I won't say anything about the helmet, except that you should now throw it away, or make sure you never use that one again, as it has given you your money's worth!
Luis