Help! I'm loosing my vocal agility. How can I get it back?

Wow, you have a trill! I can guess that you can easily maneuver through "Bel raggio lusinghier!" Given your age, it seems that your voice has not fully settled yet and no longer having the agility may be part of the maturing of your voice. If you haven't already done so, how about starting the coloratura phrases at a slower tempo before singing it at the written tempo, which is what I did when learning those tiny notes in the roulades. You can also consult a voice teacher who can listen to your voice and offer advice.