Hi & welcome,
Firstly, for safety's sake, one very basic requirement you need is to have a secure place to keep a horse. If you didn't notice she was gone for many days, then I don't want to assume, but that sounds extremely... Lax to me.
Secondly, the way you put it, sounds like she wasn't trained when you bought her & you're not knowledgeable but trying to do it all yourself? I absolutely agree you NEED an experienced trainer/horseperson(maybe owner down the road, or your farrier or vet can suggest someone) to both teach you how to responsibly care for your horse properly and to help you train her. In the meantime it sounds like she is probably safe & happy where she is, so for safety's sake(yours mostly) leave her alone until good help is on hand.
horses naturally live in groups and don't tend to feel secure(or happy) left alone. I don't personally think it's fair to keep them in solitary confinement.
'Herd bound' or 'buddy sour' or 'barn sour' means the horse is afraid or really unhappy about going off alone. Entirely natural to a prey animal that finds security in groups, even if being ridden(or whatever) isn't inherantly unpleasant to the horse of itself. You need to *earn* your horse's respect as a worthy & reliable leader who they can trust & enjoy going out with.
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