I would use these words (difficult/challenging and for/to) in this way: - This work is difficult for me (to do). - This work is challenging for me (to do). - This work seems difficult to me (in my viewpoint). It may not seem difficult to someone who is accustomed to it. - This work seems challenging to me (in my viewpoint).

The meaning is the same for both but there is a slight difference in semantics. "for" describes the state and "to" describes the view of our perception. Take this example instead, "For many people, water is wet" - when experiencing the feel of water. "To many people, water is wet" - in how we view/perceive the property of water.

One is correct and one isn't.

Technically "for" is concentrating more on my reaction, and "to" is concentrating more on the difficulty of the work. But in the end they are options and both are commonly used for the same idea.

The correct formulation is, "The work is challenging for me." You can also say, "The work challenges me."