How were you throwing, and at what, when you broke it? We can throw to hit the target, or we can throw as hard as possible, a very hard throw can break something way faster. Also, a round taken from a stump is a much more forgiving target than a big dead tree. The round, placed with the end grain as the target, will allow the hawk/axe to bite in better, and absorb more of the shock. But, a standing tree is hard on hawks, they don't offer much forgiveness.
Not all wood is created equal, you may not want to throw the best looking piece of wood, yet, you want the correct grain tightness and alignment, in most cases. The best thing to do here is
a) throw at the best target, throw enough to get it to stick, don't try to kill it, you'll only kill your axe handles. Make each throw one that is focused, and at the correct paces away from the target.
b) get several more wooden handles, go to the local hardware store and hand select your handles, make them fit, or order more through CS.