Splendour in the Grass by William Wordsworth
What though the radiance
which was once so bright
Be now for ever taken from my sight,
Though nothing can bring back the hour
Of splendour in the grass,
of glory in the flower,
We will grieve not, rather find
Strength in what remains behind;
In the primal sympathy
Which having been must ever be;
In the soothing thoughts that spring
Out of human suffering;
In the faith that looks through death,
In years that bring the philosophic mind.
No matter where you are in life there are comfort and joy to be found. The nature of the comfort changes as the years roll along, but rest assured, there is no magic age at which the thrill of living is gone.
The key is not to dwell on that which passes away, be it a valued capacity such as strength or stamina, but instead to focus on the full use of what is presently available to you. Radiance is absolute, not relative, so there is no point in comparing you now to your former self or to the capabilities of another.