Used to have the same problem. I've found it's gradually disappeared as I gain more experience composing.
I think what helped the most for me was learning to take longer in the first parts of the piece. Spending more time introducing the idea(s) automatically means there is more to play around with and eventually resolve.
Also, speaking from the point of view of a more-or-less classicist (this may or may not apply to other genres - I don't know), sometimes starting with an established form or structure can help regulate the overall arc of the piece, too. In June I came across a really nifty piece idea in 3/4 and decided to turn it into a piano minuet; this gave me some degree of control. ('Okay, so the idea needs to be eight bars long, followed by a somewhat altered version of the idea for the next eight bars, and then a new idea for the Trio here, and then...')
So yeah. Play around a bit with the idea(s) in the piece and see what new forms of it can still be added on. Above all, I'd say don't worry about it too much. Just stay open to...well, yeah, potential additions, and see how it goes.
Best of luck, and welcome to YC!