I've made a monumental error

Now that I've recovered from this terrible shock if like to ask a further question. I seem to recall this was a telephone complication. There is no way on earth I would have answered NO if asked the question about having a private pension. I seem to recall the person asking the questions said that as I was claiming contributory benefit it wouldn't take s long as he wouldn't need to ask so many questions. Could it be that he never asked the question and the answer was just assumed to be more. I'm not clutching at straws, I will pay the money bank but I do not understand how this error has occurred and just need to reassure myself I'm not going mad or senile.
Thank you.
Still in hospital and still devastated so please be gentle,

I'm now home from hospital and have heard nothing, I have sent 2 letters and 2 emails as well as the initial phone call. I'm still in shock at this error and I know it's my fault and I will pay it back but I'm still shocked at how this could have happened. Is it worth asking for the transcript of the telephone application? Do they ask questions prior to the telephone application to establish if they need to then ask all the question on the form eg if its contribution based do they say well as its contribution based I dint need to ask you about your capital or income as its not means tested? I know you will think I'm clutching at straws but there is no way I would have answered no to a question on the receipt of a private pension. Plus of course the numerous phone calls I made a few months after my award to ESA for advice on whether ESA was taxable because I had been awarded a private pension - yet none of this was picked up. What makes it worse for me of course is I worked in the housing benefit office for 25 years yet I genuinely believed contribution based Esa was not means tested. I know in the housing benefit office if a claim came in from someone who had worked in for example a local authority for many years and had finished in ill health, the fact a private pension was likely would have been flagged up and the question asked to the customer again, if it had not been declared. I'm just perplexed by it all.

Hi all, well it's now been 5 weeks since I realised my error. I'm still devastated and can't believe this has happened, I have sent several letters and spent a long time on the phone but to no avail. It seems to be complete chaos. I was told my paperwork had been scanned onto someone else's claim and then I was told that even though I send stuff recorded delivery, that was no guarantee they would receive it.
I don't know how to progress this - I just want to pay it back and get it done with. I think I'm still entitled to some ( I understand they disregard £85 of any private pension and then 50% of the excess is taken off your Esa entitlement ) . I've sent in my latest proof of pension but have heard nothing - all that has gone into my account is a £10 Christmas bonus ?? In April!!
I really can now appreciate how the DWP pushes people over the edge, I'm so glad when I worked as a benefit officer for 25 years in the council, I was compassionate and helpful! Anyone got any tips how to progress this? I have been told they don't do face to face. Yu can't speak to a manager and they promised a call back which didn't materialise, I'm recovering from a perforated bowel after repeated surgeries, I'm not in a good place physically or mentally.
Thank you