Hey relax, your post sounds so stressful lol. You should be mainly avoiding guardians until you're a bit higher level anyway. If you just move behind any object they'll instantly lose you.

This was after loading an old save bc we got stuck on an island due to the genius idea of not letting horses swim but having water everywhere.

Okay but this made me actually laugh. I can't. There isn't even water everywhere but yeah.

Listen, you don't need to the horse to travel anywhere, the game is suppose to slow moving. Yes you do need to register it, just use that as a lesson for your next horse. If it takes you a while to make it to a stable then it takes you a while, why are you rushing it?

Also, what kingdom are you talking about that you locked yourself out of? Do you mean hyrule castle? If you just started you shouldn't really be going anywhere near there anyways.

Seriously tho, just abandon your horse. If it's already registered you can just pick it up from literally any stable. If it's not then just tame another horse. It's not a big deal.