You are most of the weight, so even if you get a lighter bike it's not going to make much of a difference. Weight doesn't matter on flat ground, but matters a lot when climbing hills, which is why you do better on flat ground than on hills. You mentioned your weight, but didn't tell us your height, so we can't tell if you're overweight. If you could drop a dozen pounds it would make a big difference when climbing. Also, when climbing out of the saddle, it's easier to do if you are using clipless pedals which provide full foot retention when pulling up on the pedals.