I have the same issue, but got good advice here: Get out and ride on that mountain bike! Ride on sidewalks, grass, gravel, dirt roads, and keep riding. Eventually you'll end up on a trail.

I also went to a skills clinic offered by REI. That helped a little.

My husband and I went out to a local state park with trails ranging from easy to hard. He went out on the hard trails, I went by myself on the easy loop, and we met back at the truck.

I am still a noob, and I still screech on sandy bits (I fell really hard on a sandy road in Martha's Vineyard and it took a long time to heal my disclocated shoulder), I still avoid logs (I went rear over handlebars trying to get over a log), I still walk on most *sweet* downhills and while I'll attempt an uphill, it is rarely without unclipping and having to stop halfway up.

I don't know if that helps, but just know, you are in good company.