OVERCHARGED AS A FAMILY PLAN WITH ONE LINE!!!!

So I called T-Mobile to see about lowering my bill. I have been paying $140 with a discount of -$10 for auto pay. I find out tonight that I have been paying for a family plan with ONE single line SINCE MAY!!! 
If we do the math here, that is 275 extra dollars T-Mobile has charged me since May. The lady said I should only be paying $85 for one line. When you do the math that's where the $275 comes in. If you do a little bit more math that $275 equals out to a few bills. I do not feel I should be charged for the next couple of months. 
 

I have been with T-Mobile over six years. Now I wonder how many times they've overcharged me and took advantage of a customer that has been loyal to them.