Sometimes you need to reverse your animation — no need to rebuild the whole thing from scratch.
Steps to reverse animations
Select your keyframes, right-click, and choose Keyframe Assistant → Time-Reverse Keyframes.

Your keyframes flip in order, so the animation plays backwards.

Press Play to preview the reversed motion and make sure it looks the way you want.
One more thing
To flip an entire clip instead of just keyframes, select the layer and press Ctrl+Alt+R (Windows) / Command+Option+R (Mac).
Related how-tos
How to add keyframes in After Effects
How to fade in and fade out layers in After Effects
How to use hold keyframes in After Effects
Need more help? Explore all our How-tos in one place.