[DharmaPolice]

>1. You haven't done it before. Not exactly the same thing. You think you've done it before, but you haven't

Not to mention, even if you have done the exact same thing before it's unlikely you'll be satisfied doing it the same way. What % of projects are finished with everyone thinking "Yeah that was done perfectly there's nothing I would change if I could start from scratch"?

[TeMPOraL]

Not to mention, you're still just learning. You don't get proficient in anything by doing it once. After you do something a few times (in relatively short succession - enough for the sequence of actions to stick around in your memory; compare: spaced repetition), only then you can have a good feel for how long it takes (and you have a baseline against which to compare your ideas to improve the process).