I ran into a similar issue when I was starting Great Haven two years ago. I had a lot of experience jumping right into an adventure without much planning, spinning idea after idea and running with them until I got bored or unmotivated enough to let it fall on the backburner. I hated that; I always wanted something I could finish.
So when I decided I wanted to do Great Haven, I planned a lot. I planned as much as I could, in fact - I wanted to have as much prepared as I possibly could so that I could have a lot of material to rely on once the ball actually got rolling. Then came the question, "how much planning is enough?"
It turns out that there is such a thing as too much planning, especially if it's getting in the way of "doing." I asked in the irc when I've prepared enough, and I was told by Schazer and some others to just go for it. And they were right, I was too worried about planning where things should go that I wasn't busy actually writing the story I wanted to write. So I started Great Haven that night.
It went well, for a few weeks, and then I started to fall behind my expectations. Granted, my expectations were poorly defined (Try to update once a week I guess? Or, okay, every two weeks) and so my update schedule was poorly defined, and so on and so forth - I ended up in a multi-month hiatus where I would only update if I had the sudden urge to do it, and unfortunately, not even thinking about it anytime else. I think I could count the number of updates I did in 2016 on one hand.
During that time, I did a lot more fleshing out of my world, just by thinking about where it would go, or talking about it with friends - which means my original vision of Great Haven was very different from what it is now. You'll find that you'll actually do a lot of planning as you go, anyways, so there's no real need to frontload it all before you even start.
So I guess, while planning is important, it's not as important as doing. If you're worried about falling behind, then set rigid deadlines for yourself - Fogel updates every day, I try to update every Sunday (even if I'm consistently late.) Get used to writing and pushing updates on a schedule. In the beginning, you'll definitely feel like you're just bad at it - but once you've trained yourself to be consistent, you'll feel much, much better about yourself and your work.
These things may not apply this way to everyone, but they've definitely helped me.