An ending becomes easier when you stop asking one project to contain every good idea you had.

Projects collect ambitions the way a house collects boxes. Every good thought that arrives during the work gets added, and eventually finishing would mean abandoning a dozen ideas you still like, so the thing never closes. But those ideas are not dying. They are simply not going in this one.

Make a list of everything you wanted to include and did not, and keep the list somewhere you will find it again. Then finish what you have, at the size it actually is. The list is what makes the ending bearable, because nothing on it has been lost, only postponed to a piece of work that suits it better.