Mango Just Ruined My Weekend Away

Mango forced me to change my flight but led me to believe that it was my outward flight. Once I realised that I changed the wrong flight I asked to change the return flight to the later time. Mango said that I would have to pay for the change as I only had only one free change. I argued that surely the first change was forced on me due to a schedule change and therefore this change should be free. I asked how much the change would be so I could change the flight but they said they would listen to the call and get back to me. Nearly 24 hours later I had to chase for a response. They kept me on the line for over 20 mins and made me listen to the call after which I argued again that it was the my 1st change and should be free. At this point Guest Services informed me that in fact the later flight was full and so I couldn't change it anyway!!!! If they had let me change it the day before I would not be coming back from a weekend away on Sunday lunchtime but having a full day on the beach. Thanks Mango you just ruined my weekend.