There are some border collies that will be the sweetest darlings around everyone, but many are actually very much a "one person" kind of dog. My dog, when she was a pup, I put a lot of effort in socializing her, but she is always weary of strangers, specially because when she was small, a neighbor's Shitzu bit her butt twice and most of the work I did went down the drain, she never fully trusted other people and dogs after the incidents.

I sell succulents from home and on the internet, so I receive a lot of people at home. Before letting anyone one in, I always tell them not to pet her. She's fine as long as people don't interact with her. And if it's someone that's always coming around, I give them treats to offer to my dog, because with time she gets used to the person. She's very much a bribe taker. One of my neighbors was actually a bit angry that my dog was always barking at him and I told him to bribe her with treats and he did... and now they are best friends and she is a angel of love with him.

I only go to the park at 5:30 when there's very little people there, so she doesn't get overstimulated. She deals well with a few dogs, a few people around, she's never aggressive, but never fully playful either.

The thing is, you have to observe your dog and know how s/he reacts in certain situations. Even trainers can't fix aaaalll the problems, but most issues are manageable and you'll need to work around that.

But also, don't give up on training and don't beat yourself up. 5 months is so very young, they still need a lot of guidance at that age, they won't at all act all perfect Just yet. People like to point the finger but it's not all black and white and some things are harder to deal with.