There are many different positions that someone interested in mental health work can work in. I don't think it matters how much education one has in this field, a mental health worker job description would be one that is hard to describe.
Only years of training on the job and working with people in this capacity can really prepare you for what you might encounter. My sister works in this field, and one day is never the same as another day.
There are many times when she feels that her job is rewarding when she can really help someone make major changes in their life. There are also many frustrating days when she feels like she is getting nowhere and wonders if anything will ever change.
It has certainly been a challenging job for her and one that she plans to continue working in. She is currently working on getting her masters degree so she can be involved in more of the administrative end of things instead of the day to day contact.