I tend to have the same issues, as I've only recently gotten into mountain biking. I'll be curious to see if anyone has some good advice on it, though I imagine much of it is simply getting out there and convincing yourself to go just a bit faster, to ease up on the brakes a bit more, to lean into those turns a little bit harder, and so on, just a bit at a time. With each ride, one is bound to improve, right?