Try choosing game jam which lasts longer, a month at least. That's how I get used to participating in game jams. It was hard at first not because of ideas but because of actually completing the submitted game to my standard.
I had issues like you about implementing my idea and it costed days of jam time but fortunately I could wrap the game in remaining days.
TLDR, choose gamejam with long time limit and try making good complete game for the jam.