Accept that it's inherently an iterative process, then you don't think 'sunk cost'.

And then 'getting rid of code' is the most wonderful feeling in the world.

Getting rid of unneeded code is like clipping toe-nails that are way too long. (It must be done!)

It's like cleaning a disgusting floor in the bathroom (it can't wait).

It's like fixing ugly grammar and spelling mistakes.

It's like sanding and painting that old fence the really needed some attention.