The reason we hold onto people God is telling us to let go is because we always think it is okay to part ways only under terrible circumstances. We want to find wrong in people that isn't there the moment God doesn't approve of them.

Someone can be the nicest,most authentic person there is but not fitting into your purpose and the place God expects you to be in at the time. Actually their goodness might become in itself a barrier that blocks you from seeing what God is doing and following suit. God will remove them not because they are bad people.