Legally speaking, has it been 7 years or more since last contact? If so, they have no legal leg to stand on in forcing you to repay.
Next, is it the actual person/peoples/company that you got credit from? If so, you may want to pay them. Negotiate payments if you need to.
However, it is usually a debt collection company who bought your debt for 10¢ on the dollar. If so, make them prove the debt to you, and then negotiate for settlement. They are vicious people. Do not give them anything. Negotiate them into good terms. Do not feel guilty for these horrible people.
About the past.
Mistakes are always made.
There is no avoiding it, even with really good bookkeeping.
So, you must forgive yourself.
Then decide what you are going to do.
The best thing may be to ignore the email.