Especially when you’ve never done it.
And really, everything you do is something you’ve never done.
We feel that days are the same, but they aren’t. Monotony is a frame of mind, a passivity to pattern.
So when you say, “I’ll do this by time X” you’re expressing a wish or an intent.
The important thing is to keep doing, and let the deadlines you set float by if they need to. Better late than never isn’t an excuse, it’s a necessary compromise with a world of nearly infinite complexity, very little of which falls under our actual control.