Nagging is “See what you are not?” Support is “Look at all you can be.”
In both cases, you want to help your spouse to become a better version of themselves. It’s your mindset and the words that come from that mindset that makes all the difference.
Nagging comes from the mindset that you don’t think they are competent or have things under control (perhaps it is subconsciously). The words that follow sound accusatory or condescending. “Don’t forget to do X.” “You need to do Y.” “Why haven’t you done Z yet?” “Don’t eat that!” And the tone makes it worse. Nagging brings on a vicious cycle of complaining and withdrawing, feeling hopeless and controlled.
Support comes from the mindset of caring for the other person. You want them to have the best life they can. “I know you are trying to live a healthy lifestyle. Would it be helpful to you if I prepared a salad everyday?” The other person then feels that someone has their back, that someone will encourage them through the hard times.