MISLEADING SALES REP

I called T-mobile to lower my monthly plan. I canceled 3 tablet internets that I was paying promotional $50 / month for those 3 tablets and also changed my plan to get a lower rate. The representative canceled  the lines, changed the plan and told me that I was all set. Here I am so happy that I deducted a chunk of my bill. 2 days later I get a notice from T-mobile that I owe $142.00 extra on the bill. I call and they tell me that I had to pay Partial plan charges because since I canceled the plan I had to pay the "prorated" charge which actually cost $65.00 each. Excuse me??? The representative mentioned NOTHING of this. He happily changed my plan and went about his day. Do you not think that is something worth mentioning? They went and gave me a $40 credit due to the "inconvenience" and apologizing wildly for the rep and that they apologize he did not mention it but there is nothing more they can do. I spoke with a supervisor and she was not of much help either just said the same... "we apologize the rep did not mention this". T-mobile reps apologies is not going to help me pay this bill.. I should not be penalized for the $140 charge!!! And I want it removed from my bill ASAP. Please, what can be done to get this bill resolved? I literally am in shambles because I have been so blindsided by this. I have been a customer for more than 10 years with this company and I feel like I got completely scammed there and it is all because a REP did not mention any of this before me giving the okay to cancel these plans. Someone help please.