Working tax sickness

I’ve been off work due to work place busy, cancer flare up and mental health and depression suicidal and break down, the main issue was work place and it’s taken till last month for them to put me under a new manager and m due to go back anytime soon. I’m also a carer for my dad so have reduced hours to allow carer allowance and his care. I had a severe breakdown last year and had be told I could still claim wtc. When my ssp ran out I looked at esa but it didn’t seem I could get much because of the wtc. I haven’t has any renewal pack or summary and my information hasn’t change other than I’m not been getting work sick pay. so assumed they usually get all the information from my work anyway and send a summary but I’ve not even had that, I didn’t apply for esa so have only had my wtc and carers to live on. I wasn’t aware as I wad told I wouldn’t get esa cause of the wtc that they finish when ssp does. I now received a letter wanting to review and been informed I should of stoped them and started esa. I’m going to get a massive overpayment statement and now I return to work probably won’t be able to boost my hours with them. Had I know I would of been claiming esa but I can’t back date that now I don’t think as going back to work and even then it would only be for 3 months when I owe nearly a year. I haven’t be trying to defraud the system and esa is actually more than I get wtc so I would of been better off. I’m so stressed and upset and had no renewal pack ets to remind me of these rules and when suffering from such severe depression and anxiety you don’t think to question what employers are saying. My doctor can write a note expressing how ill I have been but I still don’t know if that will allow me the year esa to cover the overpayment of wtc. Any advise would be amazing as everyone but me seems to know about the 28weeks period