Plain text files are cheap and occupy basically 0 space. Whenever I have an overfull todo list - let's call it todo.txt - I copy it to a file named x.txt, and trim it down. Then when I finish that, I go back to whatever is left in todo.txt.

Now pretend that I cut and cut but can't bring myself to reduce x.txt to less than, say, 50 items, every one essential to complete by today. What do I do then? I copy x.txt to y.txt, and reduce x.txt to just what I plan to do for the next 4 hours. If that's still too long, I copy y.txt to z.txt, x.txt to y.txt, and reduce x.txt again. You could always start lower in the alphabet (a.txt) if you want more "space".

You get the idea. The point is, with text files, if it's got too much in it, create a backup version of that file and cut it down to size. Repeat as necessary until your todo list is manageably long.