Quoting Thrax
Your BMR tells you how many calories your body is burning. The first step in any weight loss plan is knowing that value. Your BMR determines how you exercise, when you exercise, what you eat, when you eat, everything. Not knowing your BMR is like trying to drive without knowing what a car is.
I know the website is out of date, but I was linking it to show that there are tools out there that can determine the biggest factor in any plan you'll develop. You should really look into it, because it's the difference between struggling pointlessly and succeeding every day.
BMR and all that goes right out the window when you thyroid doesn't know how to process food properly. Your thyroid is what regulates your metabolism. I think it's not a leap to suggest you
are eating a great deal more than you think you may be without consistent exercise, but your body is working against you somewhat.
You should seek professional advice from your doctor or a nutritionist. I'm not really in a position to direct you when it comes to this stuff.