Grade B is much higher in nutrients. Try to get used to it, it's better for you.
And, in 12 glasses of the lemonade at that concentration, you are consuming 1200 calories per day. That's an ok amount of calories for someone seriously cutting back to lose weight on any normal diet, but you did say that you have to eat less than most people just to maintain your weight... So maybe cutting back on the maple syrup would be ok for you. You should still get enough nutrition, especially with grade B syrup.
Personally I don't like that much syrup in the concoction. I've probably only been consuming 6-12 tablespoons per day, and i feel fine. If I feel weak, sometimes i just take a tablespoon or two of straight syrup and i feel better. (that way i don't even have to put it in the lemonade, i can keep it seperate, and i like that taste of just lemon water with cayenne better. it all mixes in your stomach anyway.)
If you continue to not be able to lose weight, try cutting back on the syrup. I doubt you'll suffer from it. If you get dizzy, just go back to the normal syrup concentration.