This is a fact of life. Handles are usually made of wood. Wood is not as strong as iron in its capacity as a handle for a hammer. Weather also influences wood quite a bit more than steel. People have soked handles in water, oil, and anti freeze for a quick fix, and I have, too. But if you want a proper hammer, install a proper handle in a proper hammer. Learning how to make, use, and maintain tools is one of the most rewarding things about being a blacksmith.