I think you are very likely to be write @NoviceVillager . The trouble is though it is something he is 'just' doing, it feels terrifically unjust.
I did my utmost to be a good step-parent - the normal domestic slog of cooking and cleaning up and driving about. There were plenty of good times. In the less good times I did my best to be flexible and tolerant. I left a lot of the difficult stuff to his Dad, but my stepson seems to have picked on a couple of incidents when he was quite young when I got quite 'normally' cross - and used them to create a narrative of my being a harsh and abusive stepmother. The fact that he appears to have shared this narrative with his partner, has put me in a difficult position.
(My partner is a loving parent, and if I had ever been anywhere near the sort of person my stepson is describing, our relationship/marriage would not have survived for three decades.)
I had a father, who now would probably be described as someone with high-functioning autism. He had meltdowns in which he hit me repeatedly round my head, culminating in a truly terrible assault when I was 17.
I am reacting inwardly to my stepson's verbal onslaught, very much as if my father has come back to life. I just want to protect myself, and do not feel my relationship with my stepson will ever be the same.
Unless he gives me a real fulsome apology in which he shows insight into his own motivations. Which at present seems rather unlikely.
I just feel as if I spent 30 years of my life helping to care for someone - and to an extent setting aside my own longer term interests - who is behaving towards me with incredible animosity.
And it's not possible for me to see the baby. I last saw the baby when they were six weeks old. That was three months ago.