Any chance you're holding your breath when you jump? I get tired easily too. It could be allergies, vitamin deficiency, low iron...pretty much anything. As for the legs, first make sure your stirrups are short enough. If you don't have enough bend in your knee, it's hard to keep you leg on. It's hard to tell without pics, but most of the time a lower leg swings, it's because the rider is pinching with the knees. Take your knees off the saddle and really try to concentrate on hugging the horse with your calves. About 3 strides out, think bear hug. It worked great for me.
You just have to see your distance...you don't have to like it.