Now comes the tedious part (but trust me, it's not as painful as other methods of doing this). On your timeline, you'll need to right-click on every last frame on your text layer between your first and last keyframes, and then select "Insert Keyframe" to make that frame a key on its own. If you try just copy-pasting your keys, you'll end up with duplicates of your text on every layer, and you'll have to just go back and delete it anyway.
The reason that we do it this way is so that we can go through and edit each frame independently without it affecting the other frames. We're going to progressively delete text on each frame, and if we left it as it was, then the deleted text would stretch over the entire animation.
