Hi OP, you have my sympathy.
Prior to having DS I had two cats who were happy, sociable, lovely little things but when DS arrived everything went rapidly downhill.
They were so stressed and they urinated everywhere, all around the house, in the car seat, in the cot, over DS's clothes, it was a nightmare. They started scratching our furniture and carpets, they became very recluse and they were clearly unhappy.
Me and DH tried everything over the course of two years to try and ease their stress but nothing made any difference. We had the Feliway Plug-Ins, the Feliway Sprays, the calming dry food you can buy, the calming tablets and drops that you can add to their food and provided them with their own 'safe space' in the house but their behaviours just continued.
We were back and forth to the vets with one of them at least once every 6 weeks because she had stress induced cystitis and that was costing us about £50 each time and on top of all the products we bought over the course of the years we must have spent into the thousands of pounds trying to address the problem. It reached the point though where the Vet told me that each time the cat got cystitis she was damaging her kidneys that little bit more and she would get more and more prone to health problems as a direct result. I spoke to the Vet at length about the problems my cats had with their behaviours and everything we'd tried and he suggested that if after two years my cats were still so unhappy then we should try to rehome them.
I was really upset at the thought of doing it but I was also upset at the thought of my cat's living a miserable life and the fact the stress they were feeling was putting their health at risk.
We advertised and rehomed them privately (they were 7 years old) but after two weeks the lady returned them because they were persistently urinating in her house too and scratching. She'd also had to take them to the Vets and pay out for antibiotics because another bout of cystitis had arisen. I reimbursed her for that though.
Following their return we re-advertised again and this time in the advert I was very honest about why we were rehoming and detailed what behaviours we had observed but that beneath that they were lovely cats and they just needed a home with people who could dedicate their time to making the cats feel loved and secure in their environment. I was contacted by a man the next day who alongside his main job he took in 'problem cats'and rehabilitated them (for want of a better word) and said he's like to meet us. To cut a long story short we did give the cats to him and his wife and two years on I'm still in touch with them and they regularly send me emails and photos and it's clear how much they love the cats. They said it took a good 3 months or so for my cars to settle in and for their stressful behaviours to cease but they were now happy little souls again.
I was so upset about rehoming them and I missed them a lot but I know it was the right decision for them.
As has been said though, the difficulty you have is the cat's age and that probably will act as a barrier. Do you have any childfree family or friends that could take on the cat? It may be worth seeing how she fares in a different, calmer environment before looking to PTS if that's another option you have been considering?