What sort of mental illness is this??Watch
Being really upset and worried in the present and just wanting to go back.
If you think its more than just reflection it would be good for you to speak to a professional. Nobody here can diagnose you with anything.
I came across a very similar post, several months ago. Was that you?
I took a course on Bach flower remedies and learned that there's a Bach Flower Remedy called "Honeysuckle" which is for exactly the state you describe.
"People in a Honeysuckle state feel that their best days are behind them and that there is little to look forward to, and as a consequence they prefer to dwell on past happinesses.
The remedy helps us to learn from and recall the past without needing to relive it, so that we can progress into the present and take joy from today and tomorrow."