Good job on getting right back on! And nice war wounds, bear them with pride! Have you thought about wearing arm and leg pads? When I was first starting mountain biking I found that wearing leg and arm pads gave me a little more confidence and a lot less scrapes and bruises. They strap to a hydropack pretty easily if you don't want to wear them on climbs.

About the helmet - replace it as soon as you can. Not because it may have hit the ground, but because it doesn't fit. In my opinion, a helmet that doesn't fit properly isn't going to protect your head properly. And you only have one of those - isn't it worth the cost of a new helmet? I've broken two (properly fitting) helmets in my mountain biking career to date, and had no serious head injuries, only a very minor concussion from the second crash (off of a jump). I attribute this to well fitting, high quality helmets. Besides, it's always nice to buy new bike stuff.

A.

PS - most of the other people racing are probably as nervous as you. Give it a try, it's fun! I finished dead last my first couple of races, and then gradually began improving - "hey, this time I was faster than someone who didn't crash or have a mechanical..."