Update!
I haven't posted for quite a bit, mainly because I went away and really tried to sort this out once a lot of people had given me similar advice - thanks to you all, it's been great to hear I'm not alone
I'm not sure how I got my head around it after battling with anxiety for so long but it's like my mind suddenly clicked and I just decided I was going to be myself and balls to the consequences! When I say be myself, I mean it seems like I've spent my whole life feeling that I was missing out on some important secret to living and socialising, so I have always felt 'other' without understanding why. I always felt like everyone else knew what the right thing to do and say and think in all situations was, whereas everything I did or said or thought was inevitability wrong in some fundamental way. As such I went about with this hardcore anxiety that I was inherently 'wrong' somehow, so I always deferred to others and watched myself like a hawk, saying to myself 'you're doing it wrong, you're doing it wrong' all the time.
I'd guess this sounds familiar to some of you (?).
Somehow, and quite suddenly it seemed, I just started not listening to that projected sense of right and wrong and accepted myself as having problems that aren't necessarily going to go away too quickly, and especially won't go away if I constantly worry about them and try to cover them up by putting on a mask and pretending to be something I'm not. I am a bit weird, and other people may well think that, but that's okay. And given a choice between being weird and unhappy (because I'm constantly struggling to try not to seem weird) or weird and happy (because that's just how I am), then I might as well choose the latter!
Now for the past few weeks, because of that decision (and it seems so obvious now), I've been enjoying amazing peace of mind and contentment like I've never experienced! I just feel so free, and it's hard to remember what it was like before.
I know my behaviour will have changed a bit because I'm focusing as much as possible on maintaining this peace of mind by being myself... Which for work means concentrating on my work, being polite and friendly but essentially remaining somewhat withdrawn from the social aspects. Not withdrawn in a miserable way, but just not paying it much mind and being indifferent to a lot of it.
I do have some lingering concerns that how I'm acting now will appear a little rude in comparison with my previous behaviour, but hopefully this is just the transitional phase and people will readjust to my way of being. I'm not being rude, but it probably looks like I've taken a step back. If that's a problem, I'd have to step back into the world of anxiety to try and work out how to navigate the social world again, and I'm very reluctant to do that. Whether colleagues will understand in time, I don't know....?
I think my friend who I talk to about this stuff will think I've given up and I should be trying harder to make friends here at work because that's what she would want to do.
But I think she lives her life playing what I now believe to be the neurotypical game of pinning your sense of self worth and happiness on the opinions of strangers and then basing everything you do on trying to secure positive opinions from them. I'm happy for the first time in my life with how I am. I can go about my day being happy like this, so why would I want to give away my own power to be happy to someone else?
Thanks again for all your input and support, I wouldn't have been able to get to this wonderful point if it hadn't been for the kind and helpful messages on this site