In my opinion, anyone working 50+ hrs a week caring for a young child would not be human if they didn't experience burn out.
Caring for a young child is very isolating, repetitious and physically/mentally draining. Sure, there are many rewards as well, but it is tough work.
You are constantly working hard to keep your charge entertained, safe, happy, clean, well fed, well rested + educated. This is an enormous responsibility here.
I am not sure if you are allowed to drive your charge anywhere, but if you are I would suggest asking your bosses if you can plan your daily outings. This will give you a chance to switch things up and do something new and different every now and then.
Also, I highly encourage you to join some playgroups w/other parents and/or nannies so you can have the opportunity to interact w/other adults from time to time.
Good luck. Hope this is helpful.