The force is strong in this student. In these both students.
Honestly, after reading shadow_archmagi's inspired reply, I can't think of anything else but approval. If you haven't done much writing beyond worldbuilding, and after some time it grows stale on you, I suspect you just lose motivation because you don't really feel that you're getting closer to actually writing the story. Which means that you probably need to do some writing challenges or that course in college, which is going to contain plenty of those, I reckon. Hopefully it'll make a motivational breakthrough for you, as you'll know that you definitely can write something and finish it, what's more important.
Otherwise, getting to physiological reason of losing interest in an idea can be helpful. The brain lives on new impressions, it's well known that writers that got somewhere don't just sit on their asses and type day and night, but travel around the world, get involved with interesting people and organisations. It may be a part of their nature, but it may be a part of their professional approach to their job. They look for ideas, and their brain looks for new data. So, if there are no more sparks driving your worldbuilding onward, and you know there are things that could use some work in that world, you might want to go out to some interesting place. Like, if your world is ancient history based - go to a museum. If it's space based - find an astronautics museum. Well, probably the simplest solution, anyway. I suspect you can't do it right now, so I don't suggest traveling somewhere.
Oh, also, if you go to a museum, don't drive, take a walk (may depend on the distance). Brains like oxygen.
About the last piece of shadow's advice, I hope you'll excuse me if I say that taking a break is actually for losers. You've got a good chance that you're going to come back to your story or world intermittently, but never really do anything with it. And it doesn't improve either your motivation to write it, or your self-esteem (from personal experience).
Finally, a disclaimer: I'm actually not the best person to seek advice on this matter, because all of my (relatively) big things that did or didn't come to a conclusion, I started writing without any worldbuilding whatsoever, and all the worldbuilding I did went into stories that never saw the light of day. Although this year I plan to change it (and I plan, and I plan, and I plan).