I can’t play your game (it’s Windows-only) but having submitted to 23 of the 24 jams I’ve joined, I feel like I can comment on actually finishing things.
In general, you want to know what the “core” of your game is. What is the thing you want the player to be doing most of the time? Start with that, with no real assets. Personally I’m often making movement-focused games, so I’ll start by making just the physics in empty rooms with boxes for characters.
At that point, you’ll either have something that’s already fun, or you have nothing. Make a snapshot. (You are using version control right?) From there, you can add features, but always make sure you have a playable game. Snapshot frequently, basically whenever you get the next playable thing. If you have a list of “NEED THIS” and “NICE TO HAVE”, focusing on the needs first, you’ll be fine.
When time runs out, submit that latest playable version, that’s your game!
All that is on planning. Estimating timeframes is always going to be significantly harder. That’s the kind of thing you can only get with experience, and even then the typical recommendation is to double or triple everything you imagine. For instance, in my submission I have an achievement system, and I was sure that I’d be able to fill out a list of 48 achievements in a few hours. But the reality was significantly beyond that, and as a result some things just didn’t happen. This is fine. 72 hours is not a lot.