I'd look into a used bike.

Mtn biking is a tough sport and you could hate it or you could love it....before spending bucks on a bike you won't use I'd make sure the sport was for me.

The problems you listed in the first post are not so much bike related but rider related....it's probably just inexperience.

Good luck in whatever you decide...fwiw...I absolutely LOVE mtn. biking and it's a great workout.