Border collie puppy, nearly 5 months old, is on a special diet prescribed by the vet so can't have training treats and it's hard to keep her focussed/engaged with us during walks. She's starting to hurt her neck with pulling so I'm looking into harnesses. I'd really like to get one that gently discourages pulling while being easy and comfortable to use - it should grow with her too, easily adjustable. Please recommend one! Thanks!