American Airlines Charged me over $500 to get on flight and would not let me board or refund Dallas Texas Nationwide

I called American Airlines about a ticket I had for Sunday Travel and asked to change to Sat. They told me they would for a charge of over 500.00 --Because my daughter could meet me in Dallas for a ballgame and could catch a flight in Denver to meet me at DFW airport I agreed to spend the money and do it. When I got to the airport way ahead of time  and gave them the record locator they confirmed I was on the flight but kept saying "they" referring to AA Customer service "did it wrong and charged you too much so we are going to correct and chargerg less" . I said " Thanks but just dont let it cause me to not get on flight because I have a daughter who has already boarded a plane in Denver and will be stranded at DFW without me". They then got on phone with AA Home office and the two parties got reall ugly with each other while i am listenning. This went on for about an hour with them constantly assuring me it would be handled well before the plane closed door.They said they wer holding plane and gave me boarding pass only 8 min before scheduled depqarture....no one was in security line so i was at the gate in 3 min.and the door was closed. No one was at the American gate and I had to get security to get somone there.  By then American booked me for the evening flight, would not refund the money and left my daugter stranded in Dallas. American would not even apoogize, took my money for a flight chage they would not allow me to board, refused a refund and caused my daughter to spend 790 on a plane and ballgame ticket she vcould not use because she was stranded at DFW without me.I am Platinum with AA so I would like to see how they treat people that dont fly regular with them. So Take my advice Go Southwest---Never American---They will take advantage of you and will not make it right.