I had the same problem when I started riding on my Wal-Mart bike. The fact that our bikes weigh 45lbs really doesn't help. It takes a lot of leg power to get those bikes going, especially on any kind of climb. That Mongoose is a pretty decent bike, looking at reviews it seems the things to watch out for are the shoddy quality of the brakes, bending the rims, and bottoming out the forks. The only thing I can suggest is try to stick with some more horizontal terrain, stick to a high cadence (pedaling speed), and take it a little easier. Stamina will come with time.