Has my dog got a shit life?

[Truckingalong]

Young border collie. Horrendous start to life. Kindest most loyal and adorable dog I've ever known. I'm with her all day, apart from a couple of hours in the morning and evening. She has the run of the house all day (average size semi) and back yard (average size, paved, not grass). She is extremely reactive to anything outside but especially other dogs, so when we do go on walks, it's all a bit difficult and I have to pick my moments and she can never go off lead. She pulls like a train too so is in a halti. Sleeps in the bedroom with me. Is this a shit life for a dog? I worry that she's bored to tears. I kick a football around with her for a while a couple of times a day but it doesn't feel enough to me. The rest of the time, she just snoozes.

It sounds like you've put a lot of work into your dog already, she's lucky to have ended up with you because plenty of people give up on far easier dogs.
We have an incredibly reactive rescue dog who has very similar issues to your dog. She also can't be let off lead at all and my husband refuses to walk her himself because he finds taking her out so stressful.
We don't go the kind of walks that other people take their dogs but I don't think that necessarily means that my dog has any less of a quality of life to any other dog because we've found ways of working around it. We do canicross during the week which has been a godsend and really tires her out and keeps her mentally stimulated too as she learns and practices the commands we use. It's also really helped with her reactiveness to other dogs as we now also go with other groups and I feel comfortable knowing all the other dogs are on lead and can't coming running up to her so she's learned to be around dogs without them coming too close and over time has gotten closer and closer to the other dogs.
I also take her hillwalking every weekend and wear my canicross harness for this so I have my hands free. She loves this because we can be just us in the middle of nowhere with nothing that stresses her around. It also knackers her out especially after a really long day or full weekend in the hills.
We also go at least once a week to a secure field (as people mentioned previously) so she can play off lead and practice recall in the hope that one day we can use it for real.
Doing this with lots of training and stimulating games at home is plenty for her. I have had the odd person make a comment when they realise that we don't take her walking round the streets for hours every day or don't let her off the lead but I am confident that she is much happier this way and it means that we can avoid unexpectedly coming across her triggers while we work on her confidence with these through positive reinforcement.
Your dog may have issues which means their life is different to that of other dogs but that doesn't mean that it's not the best life for your dog.

Could you try Clicker training? ( you can buy clickers that you can reduce the noise of the 'click' so as not to frighten her) Fairly easy to do and once you get the basics right it can be used to teach many skills/ tricks etc.