I was a horrible public speaker and had no real experience instructing until I did my trainers course, I knew I had to work through it to get into something like this as a job which was hard but I'm glad I did it.
I'm very lucky that my boss is an awesome instructor and I work as part of a decent sized team (one of 5+ senior instructors and 4-6 assistant instructors) so we can constantly critique and help develop one another.
It's taken me a long time to get used to instructing in front of my boss, because I know how good he is and how well he knows his stuff but thats faded as I have got used to it and practiced it. My new hurdle is every time he points a camera at me to film something for a client my handling falls apart.
I've been working with this business almost 2 years, have been a senior instructor (someone that can take classes alone) since Feb 2013, I teach between 5 and 8 classes a week. My nerves dissipated over the first 6 months or so but I am constantly working at finding different ways to explain the same things and keep classes fresh and interested.
Sometimes make it fun and informal and let it flow, other times add more structure. Sometimes crack the whip, sometimes go in gently. Sometimes push obedience, sometimes push socialisation/confidence development.
You can't please everyone, I generally get positive comments on my classes but I know not everyone is going to be happy with every class. It happens, take note of the reason, consider it but don't let it get to you.
For the first few months instructing I felt like I was really getting the hang of it, then I'd go through weeks of classes feeling crap, then they'd improve, then I'd feel lost and it would go up and down until they sort of levelled out. I am currently experiencing the same thing when taking our off leash Advanced class, something I only got introduced to in the last 6 months. I'm riding out the ups and downs I feel at the end of each session knowing that eventually it will level out.
My boss has been doing this for more than 15 years and he still walks out of the odd class or demo telling me it felt crappy. It happens.
The good days feel awesome though, I love watching people out and about with their dogs and helping them progress and bond with them.