I find it hard to do something I consider worthwhile while on a spring break, despite having lots of a free time.

Perhaps it is because you have no deadlines, no compelling reason to complete something specific?

I tend to make grandiose plans — I should meet new people! I should be more involved in sports! I should start using Anki! I should learn Lojban! I should practice meditation! I should read these textbooks including doing most of exercises!

Those aren't plans.

— and then fail to do almost anything.

This is the retrospective evidence that they weren't plans.

Yet I manage to do some impressive stuff during academic term, despite having less time and more commitments.

There you are—commitments, and a structure, in this case externally imposed, once you've made the decision to go to college.

If when you have a wish such as "I should meet new people!", your thinking does not immediately proceed to considering what sort of people, where to find them, when to do it, and what the effort will look like, then it's just daydreaming.