Jetstra Error which I now have to pay extra or lose my original flight payment

I booked a return flight from Cairns to Brisbane return for January 2019 online by my mobile phone in November to see my family in Brisbane. The site was playing up at the time and would not register my booking on the first effort unless I booked baggage choice as well. I did that. It took over two hours to book as it was a cheap flight Friday frenzy booking and I guess there were many people booking at the same time with only 8 hours to book.
On receiving a reminder tonight 25/12/2019 to get ready for my flight in January, the booking now was from Brisbane to Cairns return. I called Jetstar and they were saying it was my fault and that was the booking I made and they were not going to change it unless I paid an extra $340. I was to see family in Brisbane who I hadn't seen for some time. I had booked the $40 bundle where I could change the flight if I needed to, I thought anyway, but now they were saying I could not change the flight and was only able to change the time of the flight. Because of the online error by Jetstar I was to either to pay another $340 to change to Cairns to Brisbane return or lose my money. They would not refund my original booking. So wrong seeing it was Jetstar's error.
I am so disappointed with the unprofessionalism by the person I spoke with from Jetstar service as I have never had any issues before with their service and have always given Jetstar full points for the safe flights and service I have received in the past. I will need to rethink who to fly with in the future.
I am so annoyed and upset that now I miss out on seeing my family for new years and that I lose my money on a flight that was incorrectly lodged by Jetstar on booking this flight. Another $340 is just over the top because of Jetstar's mistake and online booking services.
I cant believe this has happened. Disappointed frequent customer.