Mytrip.com complaint: Flexible ticket rip off

I Purchased a one-way ticket from Auckland NZ to Queenstown NZ for April 16th 2022 for £75.00 approx. This ticket was purchased as a flexible ticket. I soon realised that I had entered an incorrect travel date and need to fly on October 16th 2021. I called and a new flight was found. I was told to hang up and call the airline to check availability and that a connection time wasn’t too short. I then phoned back (20 minutes!) and was told that to change I would have to pay an additional £177.00 ($340.00 NZD). A new ticket from Air New Zealand (same airline, same flights, same date) cost £130.00 ($250.00 NZD). How can a completely new ticket be cheaper than using a flexible ticket that already had £75.00 paid. The attitude I received was poor. Disgusting company! I have written off the original purchase and bought new ticket direct with Air New Zealand.