Don't forget to warm up really good before the race. If the course is open you should first of all stretch out real good and then do some laps building up to a final warm up lap at race pace. Show up to the starting line with your heart rate up. A lot happens right off the start line in a cross race and you go 100% till the end so you don't want to use the first couple laps to get warmed up.

As far as being scared on descents you will most likelly just get better and better at it once you build up confidence. I've noticed for things like steep hills and bunny hops when in a race situation you will be surprised at what you can do. Especially if you are in the pack and everyone drops down the hill like a rock. At that point it's like you have no choice but to fly and then when it's over you just think, "oh ok I can actually do that". Also your fear may cause you to be more dangerous to yourself. Sometimes it is actually safer to go faster over certain objects while your fear will slow you down and be more dangerous.