Just a suggestion... what really helped me get over the "what the H*LL I'm doing on a mountain bike" fear is asked a good friend who is a strong and skillful rider, to take me out and just go over skills with me. We practiced attacking descents, climbing, going over obstacles, negotiating whoop-de-does, etc. Once I realized the bike does most of the work and I'm not going to fall and kill myself, then I got more confidence. More confidence means more speed, more speed means the bike is able to work better etc.

Tell the DH you loved the ride, and want to improve would he mind just spending an hour or two with you reviewing skills before you start a ride. Or if you come across something "scary", stop and ask for him to show you how to attack the "scary", practice it a few times and move on. He'll love and appreciate your enthusiasm.

Stick with the sport - it's just so much fun. And I've learned, even if I fall, I'm less likely to be really injured like a fall on a road bike (dirt and sand can be soft!).