A bit of back story:
Two years ago, my mum bought a ST filly off the track. She had barely been back, but my mum thought with more training she would be a good horse for her. It turned out that that was not the case. Once we Nelly had gained back her lost weight she became the typical energetic three year old. My mum wouldn't even ride her, or let alone work with her. So she gave her to me after she bought an older horse that she felt comfortable working with. I already had my horse to work with and found working both of them to be time consuming and tiring, so we decided to sell Nelly. Because not a lot of people have an interest in Stanardbreds around here it was months before anyone even responded to our ad, which left me lots of time to keep training her. During that time Nell and I bonded so far to the point that she will only do things for me. Everytime someone would come out to see her she would act like a total brat with no manners so no one would want to even consider buying her. We then decided she would probably be happier with us anyway, I sealed the deal by throwing a total fit the last time someone offered to buy her, crying for the entire night, telling my parents we couldn't possibly sell her now So we tried to have a college student come out to ride her with us, but that was just another repeat of when we tried to sell her. She'll only behave with me.
Because I want to start showing her this year, I have to put more training on her this spring and summer, so I was considering taking her to the barn I'm teaching lessons at. That way, I won't be running from township to township to work with my horse at my own barn and then teach lessons at someone else's, I'll also have my coach nearby to help. The only thing I'm concerned about is how she'll be behave being handled by other people when I'm not there. What are some things I can do about the way she acts with other people?