How can I decide if I can do that big thing in that short time?

Katiane F.
You probably can't! Things always take more time than we think because we are terrible at estimating that. Get what you can done, then either continue working on wast what you budgeted, or put it on hold.
Laureano S.
It doesn't matter if you get the whole big thing done in that amount of time. Just make sure you make some progress towards it. Just thinking about it can help, so that it never leaves your mind. That one big thing is just that – big. Realize it's probably not going to get done all at once, and be proud of what progress you make! Life rarely makes big chunks of time so you can get a whole project done at once. Dedication and like this gives you more time to find the best ideas, too. It gives you time to reflect. Don't stress about it not being done. If you regularly make time for it, you can rest assured that it will be done after a few days, weeks, or months!
Hunter W.
Take it one step at a time. And stick to a timeline plan. If you can’t, think of what’s the worst thing that will happen If you didn’t do it.