Yeah we all have been there once. Having a big idea and half way through reality hit us.
One last thing I would like to share is how i usually gauge the total duration it will take to get a game up from an idea.
I would first leave the last day of submission to be the polishing, double checking any remaining bugs and prepping the build/admin stuffs for the submission. On this day i should not be adding new stuffs anymore. That's one day less from the total duration of the jam.
Then on the first day itself is for brainstorming, locking the game mechanics and making a prototype to get the gist of the game. If it ends up being meh i can still change it within that day itself.
Next, i would roughly gauge how long it will take to get the assets up.
Then the remaining days would be code and playtest. Don't forget to add some rest in between too which is actually really important which most devs tend to ignore. If the total days add up is beyond the game jam duration i guess u know where to scope to down from there. If im in the middle of the jam i would just drop one or two mechanics/features. Else i would generate a new idea that can fit into a small game.
Alright thanks for reading.