That is great advice on evaluating your mainsail.
Sounds like your boat could use a few upgrades beyond a new mainsail.
Consider sailing the hell out of the boat in its current configuration. When you are ready to upgrade, sell your current boat and buy a different one either bigger, faster or in better shape. By the time you re-rig the boat and buy a new suit of sails, you might spend more than a NA 23 is worth.
Q: "How can I get better shape out of my mainsail"
A: "Just buy a bigger, better boat!"
Even if your sail is blown out, the advice you have been given regarding rig tuning, and sail control upgrades are guaranteed to make a difference, and shouldn't cost you very much money.
The luff rope, (aka bolt rope), and foot rope may be shrunken and need to be "let out". Converting to loose foot may be an option. It is hard to assess, based on a pic, the condition of the cloth to determine if it is worth putting much money into the sail, but if the cloth has life left a sailmaker can do wonders with a recut.
Another option is to search for another used sail that will fit.
Before any of that you need to get your boat set up with the basic shape control tools: outhaul, Cunningham, and maybe even backstay adjuster.
Post some better pics and we might be able to help more.
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