You alright have alot of sound advice about how to fix the herd boundedness, but I just want to beseech of you one thing...
Don't sell your 23yr old horse. Please. Please don't.
So many old horses go to *good homes* where they get bounced on by beginners, neglected, or simply ruined by inexperienced treatment. Old quiet horses suffer most from beginner mistakes because beginners buy them and they tolerate it, or *heaven forbid* crack and get sent down the road. You know where I mean, too.
Your horse is old. If you do decide you need a new horse, retire your current one to pasture, or lease her out locally with a GOOD contract.