I'm paying for T-mobile's mistake

When I got my last iPhone, I was offered a new apple watch by just paying for 2 years of LTE service for it. I set my account to autopay so I wouldn't miss a payment. After a little over a year I checked to see if I was close to paying off the Apple watch service only to find out I was paying for the watch equipment fee ($13.75 for 24 months = $330 total) AND an LTE fee ($12 for 24 months = $288) which would end up costing me $618 for an apple watch! I called T-mobile to let me know they put me on the wrong plan and they said I've been paying them too long for them to change anything and they couldn't refund me for their mistake of putting me on the wrong plan. I don't understand how a billion dollar company is ok losing customers over not fixing a few hundred dollar mistake that THEY MADE! I will be looking for a new phone service ASAP!