T Mobile double charged me for almost 2 years. What is a fair refund to ask for?

In December 2022, I called in and shut off an extra line on my plan (not under contract). Everything went great, rep was awesome and the line was removed from my service (no longer shown on my bill). Recently I have been struggling with funds, and realized they have still been charging me for this line (June 2024). I called in and got another great rep who fixed my bill going forward.

I know I cannot, and will not get all my overpayments back. I should have paid attention and fixed this sooner. I have been with Tmobile for over a decade, and like them and want to keep them as my provider. What is a fair refund amount to ask for? My plan should have been 55 a month, and they were charging me 104.

Edit: I had a lot of 'extras' on my plan. A watch, insurance for my devices, etc. I had them all turned off last month. This is the first month that I received a bill for just my one line. When it was double what I expected, I called in. That is how I found out I have been paying for the canceled line.

Second, edit to explain how I missed this. I started my plan about 10 years ago. It was a single line, I used my own second-hand phone. My bill was 45 dollars a month, plus fees. A few years later, I bought another second-hand phone and just added a line. My bill was around 110 120 a month after taxes and fees ( I can't remember for sure). A few years later, I received an email from T Mobile offering a free smart watch if I started a line for it for 9 dollars a month for 2 years. I wanted a smart watch and signed up, with insurance. My bill went to about 168. Had all three lines for the next few years. In Novemeber of 2022, I got a bad diagnosis and had to cut back hours and expenses. The second phone hadn't been used for over a year, so I called to have it turned off. Told them I couldn't afford it, and they could tell it hadn't been used for over a year. They were great and said it was canceled. The following month, my bill only went down about 20ish dollars. I honestly thought I kinda got screwed on the watch, and that was the extra expense. Maybe I didn't read the fine print? But I liked my watch and didn't want to shut it off too, so I sucked it up and paid the bill. Well, I recently my heath declined even more, so it was time to turn off the watch too. I did that 2 months ago. My very first bill without the watch came in and it was 120. Their essentials plan is only 55 a month before fees and taxes.i called them to find out why my bill was so high, that is when they informed me that the other voice line was still being charged even though it didn't show on my bill. T mobile confirmed the extra charge was for a phone I had disconnected 1.5 years ago. Yes it was very clear the voice line was canceled in December of 2022. It has not been listed on my bill for the last year and a half. The rep was very sweet and fixed my bill for next month to 70 dollars with fees and taxes and offered me a 50 dollar credit to my account. I do not feel like 50 is enough, but I also think asking for the full refund of 1.5 years isn't going to happen either.