There are some phrases that just "on" as part of the phrase. For example, "My mom is very hard on me." This means that the mother has high expectations and when the child fails, he gets into trouble for it. "Tough on" is very similar to this. In this particular sentence, it means that the person is a working in a field where it is easy to feel bad about oneself. It is challenging (or hard or tough) for people to feel good about themselves in that particular field. So, we say it is "tough on self-esteem." Hope this helps. Let me know if you have any questions.