Self-perception
is an interesting thing. My perception of my 'self' is based upon mental
formulations in the present moment - but they are formulations related to my
past experiences, knowledge and memories alongside my current imagination. So
any self-perception in the here and now is rather like a weighted average of
all those past events, circumstances and feelings already lived. They are
balanced out in the mind of the 'me' that makes those present day perceptions.
Regarding how stably they average out - that perhaps depends a lot on the historical bottlenecks of the extremes in life. Someone whose ups and downs have been steady in life can probably find a more stable present-tense average than people whose extremes have been loaded onto their younger years or years closer to the present. Someone in their forties whose past damage occurred in their teens may have had three decades in which to equilibrate to an average. That may be why many people who've suffered lots in childhood go on to be calming, balanced, inspirational figures in maturity. Someone in their forties whose damage has occurred in more recent times may well still be raw from it, thus aggravating the average in more immediate terms. Those are just a few possible examples of many.
That probably also explains why people who suffer extreme damage on a regular basis with little respite often end up cracking up - they are not able to obtain the weighted average necessary for a balance of experiential qualities. It also perhaps explains why those who've incurred no real damage in life, and a live a very circumspect and moderate existence, have often experienced fewer highs.
Of course, there will be exceptions and outliers, as well as patterns - and these above examples are more like jigsaw pieces than complete pictures. But I think they point to truths that are actually manifest in people's lives.