@Struckbylightning Tbh i'd be more surprised if nothing happened when a dog stayed at a holiday cottage! I'm glad Merry enjoyed herself though and that you were able to relax and trust her 😊
It's completely normal to struggle at this stage.
I know posters sometimes think I get get quite sharp with people who struggle at weeks 8-12...but I do it because it does get worse and I just wonder how they'll cope at 5 months if they can't cope at 2 months! Everyone always says teething is the worst stage. But it's not. When they're teething, they still want to be with their humans, and cling to you because they're in pain and it hurts. Once they're over that stage, and they're a bit braver and the worlds a bit less scary, they're little turds.
I would stick to the long line for now. Do you have any enclosed dog walking fields near you? There are a few near me, and one is actually a former riding school so it's covered floor to ceiling and the gate is impregnable - Jackie Chan couldn't get through it (and apparently they're becoming really common as more riding schools shut up shop). So, I'd try a dog walking field. They're incredibly stimulating given how many dogs go there - so good to wear her out - and you can walk them beforehand whilst she's in the car to make sure it's fully secure so you can try her off the lead in a safe environment.
Is she food motivated? Or is her motivation the chase? Collies do have a herding instinct, which makes them harder to train I imagine...but if you can figure out what motivates her, then you're halfway there. If you are struggling then there is absolutely no shame in calling in a trainer or some additional support.
Menopause is absolutely miserable and adding a puppy to that is even moreso, so you have my full sympathy! It's that tripple whammy of not enough, lots of stress, and dealing with two things that takes up your life 24/7 - the combination is exhausting. And don't feel guilty for thinking about getting rid of Merry - we've all been there. There's a reason Twatdog is called Twatdog. Merry will get there, it just takes time.
This may sound odd...but is Merry a humper? Or showing any signs of it? I know I joked above with @Lavenderdog about sex toys. But one of the tricks I have found is to give high-energy dogs that you can't over-exercise whilst they're young, a bit of an 'outlet.' I have a giant elephant (or duck, depending on availability) that's a childs toy that I still now shove at Twatdog on occasion when he's being manic. A little bit of toy (or cushion) humping never hurt a manic puppy - and it could well be that Merry is just at that stage where the exercise she can take isn't enough, and she needs an outlet for her energy.
FWIW, no dog is perfect. Anyone who declares otherwise is a liar. Dogs might be close to it - but there is always something that will catch their attention. And even in breeds, or in my dog's cases, families, it can be different. Eris lose all sense of decorum if she see's a ball, I have found. So if I see someone throwing one for their dog, she goes back on the lead - because there is no way I am getting her back once she see's a ball. I only know that because she's taken off twice in pursuit of balls. It's never been an issue in 20+ years, and nearly a dozen, Goldies and yet with her I absolutely cannot trust her.