Yeah i struggle with this problem too!!! That's half the reason why my games end up unfinished !!(The other half is because i scope a little bit too big)
Yeah, I scope too big as well (I wanted to add multiple weapons with upgrades for my second game jam as a solo developer lol), I think plannig the levels and the mechanics and drawing them on paper would help with this, as it would be easier to understand the project and the size of the scope ahead of time