Originally Posted by garagegirl

Yeah, assuming your bike doesn't have some underlying mechanical issues, and that your tires aren't horrid, you might want to take some classes. REI offers beginner mountain and road classes. The instructor might be be able to assess what you're doing wrong. My suspicion is that you're braking incorrectly and you're not disributing your weight correctly on decents. The handling skills you will pick up from an intro mtb course will be very helpful.