A couple of things to look at. First what is the weight of your trailer and what is the weight rating of the axles? A lot of axles have a tag on them with the rating.
Second, what kind of lube are you using on the bearings? You should pack them with a good quality wheel bearing grease.
Third, are you preloading the bearings correctly? If you over-tighten them or leave them too loose it can cause premature failure.
If none of these are the problem, I would seriously look at replacing the axles with heavier duty ones.
Are you replacing the bearing races when you replace the bearings?
Are the seals damaged and allowing moisture to enter?
