I’m not sure if there are any “outside” challenges because if you really want to succeed in this field you will need to keep improving as a trainer and a person. This means keep educating yourself and always be flexible with the changes around you. A great trainer is the one who can be a drill sergeant, a cheerleader and a philosopher all at the same time and know how to mix all three in order to connect with the clients. So you will need to find what your weaknesses are and try to work on them. At the end we can only control those things that we are directly involved with, and in my opinion learning how to let go of the those situations that we can’t do anything about it, is the toughest challenge of them all. The rest is easy.