Losing my objectivity is especially problematic when the solution doesn’t come easily and I struggle with several solutions that could work. Setting it aside overnight and not thinking about it until morning allows me to see it more clearly.
There’s also the technique of placing the design work in different contexts, which tricks one’s mind into seeing it in a different way. For example hold it up to a mirror, print it out and hang it on the wall, or look at it upside down.
My wife used to be a professional illustrator, so I’ll typically ask her what she thinks. Of course, you likely don’t want to ask my wife for her opinion, but if you have someone available (coworker, family, friend) who understands enough about design and will give you an honest opinion, it’s worth asking.
And as @Steve_O said, experience helps too.