[Smudge]

Firstly, time spent learning is not time wasted!

Secondly, the best way to avoid making painful design decisions is... to make them enough times that you instinctively remember them. There aren't many shortcuts to developing that instinct, so take this opportunity to try and generalize your learnings -- what do you recognize now that you didn't see when you started the project? A healthy amount of retrospection will make you a better engineer, and you'll also get better at identifying uncertainties and risks before you start coding.

Lastly, up-front planning and design documents can only get you so far. They're important, sure, but at some point you'll be down in the nitty gritty details, and you'll need to make unanticipated course corrections. As you gain experience you'll start being able to fill in more of those gaps on your own, but until then, you'll want to loop in other engineers more frequently.

One of the qualities of a senior engineer is that they act as a force multiplier for their team. So as a relatively junior engineer, don't be afraid to make use of your senior engineers! They should be there to support you and make you better, beyond just helping you plan out a project. If possible, pair on some of the trickier parts with them, so that you can see how they'd approach the problem and start picking up on things that might not seem so obvious to you right now.

Hope that helps!