British Airways SA is CHEATING ME

I had a return flight from CT to DBN with BA. I had to change my flight as my Mom was in hospital, so I needed to bring my flight to Durban forward. I phoned to change the flight, but it was cheaper to book a new one with BA (ironically), so I booked that and cancelled my durban flight.

I am meant to be flying to CT tomorrow morning, but when I called to add another bag now, they tell me that this flight has been cancelled for me, as didn't take the first leg of the flight???!!!! ***!? This is absolutley ridiculous. I have paid for 3 flights with BA and they are giving me ONE of them. They are absolutely USELESS.