It seems to me that OP already has many of the advantages of retirement, as she only works 3 days a week.
Only someone who is not retired could say this!
I worked 3 days in a professional job for 23 years - it is incomparable with retirement! Totally incomparable - I might have only worked 3 days, but I was still up at 6, out of house 7-6ish, driving a minimum 70 mile commute, sometimes longer if visiting site or working out of other UK offices. My days off were taken up with family life, I had very little social life as was too shattered. Any walking that I did on my days off was limited to an hours drive away, as I was always aware of having long drives on my work days.
I was always a work to live person, didn’t hate it, but didn’t enjoy it at the end, so much politics and so little support from those higher up. Changing jobs wasn’t really an option, as I was in a quite specialised role, plus had a long service with my employer.
Far from going downhill mentally, I feel revitalised, as I am no longer exhausted by work and the associated commuting.
Retirement gives people freedoms that even those working PT don’t have - the choice of which days to go walking, of booking a last minute trip without considering work commitments or annual leave, of having a chilled day if feeling a little under the weather. The choice to do volunteering if they want (I haven’t felt the need yet, but may do in the future). Not having to fit a trip to see relatives into one day, but having the option to stay a few nights.
I have to agree with @ArseInTheCoOpWindow, in that the majority of people telling Op not to finish, are still working. I have had similar reactions from one particular ‘friend’, telling me that I am too young to retire. I have come to the conclusion that she is envious.
Interestingly, all those I know who have retired before 60, and there are quite a few, told me (when I was in the ‘considering it’ stage) that they had not one regret. Early days, but I feel the same so far.