• It should be fairly simple to implement that
    It won’t.

  • It can’t be that difficult
    It will be borderline impossible.

  • That’ll take a day or so
    Three months in and you’re nowhere near finishing it.

  • It’ll be done by the end of the week
    It’s Friday PM and you don’t even know where to start

And so many others.

Stop being an optimist.
Be a pessimist and you might end up being a realist.