Unhappiness, statistically peaks at 45/46, it's when most people are at the most stress, lowest.
Usually, but clearly not all apply, career might not be exactly what you hoped, likely to still have dependents at home, aging parents, beginning of declining health if bit for you for friends, ebbing friendships for the time being because they are being pulled in all directions as well, most still have a mortgage and other huge financial commitments (uni fees, driving lessons,), the list is endless.
We reach a peak low
But it does slowly start to improve.
I found finding something, anything small that helps recharge you or even just claws back a bit of joy made enough of a difference.
I found reading novels in the bath a good kick start, increased my concentration, let me escape into stories for half hours, hour at a time, I liked the stuff I was reading so clawed out more and more pockets if time, herbal tea or wine on the go, it helped. I seemed to be drawn to female writers more, I used to be quite 50/50 male/female authors. Starter with I guess less challenging texts, and then wanted longer, more complicated, in depth (?), not really sure how to explain it, stories.
It semed to relax and envigour my brain at the same time. Which helped.
Not saying it's the answer to all life's struggles but thought I would say what contributed to riding the mid 40s depressing treadmill for me. (Not that far past mid 40s now and it did improve, external factors seem just a bit more manageable)