I think you need to set out a complete timetable cross referencing this with the proof you do have, your credit card statements, your bank account statements, your mortgage statements. And maybe on a month to month basis show how much money you had.
So say Jan 2009, income (gross) 3000, paid mortgage 1500, esb 200, car insurance 100, groceries 400, GP bill, 100, heating 200, petrol 100. Left over discretionary spending €400. Wanted to sign 7 yr old up for gaa but it was €90 and he needed boots and kit at €100 so did not do this. Left in bank before payday €15. No opportunity to save for a holiday etc. Should have been €515 as bank overcharging €500 on mortgage. And then show bit by bit, month by month, what you spent your money on, how much extra spending money you should have had and what you would have spent it on if you had it. For example, 8 yr old cousin started dance in September 2014, we would have sent Jane too but could not afford it. Brother says dance cost approx 200 per year for 6 years.
It won’t be easy, and as someone else said, the banks may not take emotional side into account of it at all, but if you can lay it out with reference to normal family expenditures, comparing your experience to you circle of family & friends and what discretionary spend they had and what they did with the money and roughly how much it cost them, they may pay more attention. So say you lads group went to London every year for a weekend but you only started going with them in 2018 because you could not afford it, then add that to the column of what you missed out on, giving it a monetary figure. Maybe go through your family photos and list where you went to Sligo for a weekend in 2009 but as your bank statements show you did not have the money so borrowed it from your dad but you should have had by then an extra €5000 that you werr paying in mortgage so would have gone to Spain for 2 weeks with your sister and her family instead.
It seems like a whole load of work to me, that might depress you greatly but it may help clarify to the bank how your lifestyle was compromised because you were paying them too much. If you had any email or texts to back this up it would help, “hey john, would love to go on your stag weekend but can’t with the 3 kids etc, see you st the wedding”. Stuff like that.
Of course the bank could ignore it but if you could add to timeline when you contacted bank re mortgage and show when money was tight you were trying to get bank to acknowledge you were on wrong rate it might help.