Your PIP award may be reviewed once or more in those four years. If you know the time between reviews you know how much time you have to see whether you can work without it having financial consequences. PIP isn't really about whether you can work. More about whether you can function independently, which you should also be able to do when you're at work.

For us, the most important factor is being able to monitor our mental state. If you can't do that yourself, which is per se the case when psychotic/manic, you're unreliable. Depression makes matters worse. When you're unreliable, you're probably not just less of an asset to an employer, but a liability. You cost more than you're worth. They could've hired someone else who would live up to expectations. Pretending to be more reliable than you are really makes you a fraud. Accepting that others might function better than you do doesn't.

BPD makes me even more of a liability.

But to be honest I feel like a fraud myself sometimes. But I volunteer at a mental health charity and I try to create a succesful company. I've done more important things the last couple of years than I'd ever done before while working as a software developer for a big company whose services nobody really needs and doing things others could do better than me anyway.

So maybe being self-employed could be a solution for you as well. And maybe you could find a more suitable place for you to volunteer (helping others with mental health problems would make you very much an asset, for example). You should exploit your strengths and accept your weaknesses (even when you yourself wouldn't say they are weaknesses).

All this PIP nonsense about how it's not about the condition you have but about how it affects you, implicitly making you somehow responsible, reminds me of Monty Python's .

We are the only group that sometimes really believes that any problems won't affect us and we're more than able to do anything, when we're not. You should be thankful that people do realise that and do consider our particular condition and not just how (we think) it affects us.