Not all jobs are for everyone. The first thing you need to decide is if this is the right job for you. It's very different to be stern with a coworker or vendor than it is with a client. I'm very stern with my coworkers and vendors, and justify it by "doing what's best for the company", but I don't ever want to interface with clients. I hate upsetting or disappointing them, and I would not feel happy at such a job.