I can absolutely see why you're upset. That's a huge lie with massive implications for you and it's put you in an awful position. I'd it hard to forgive someone for that.
As others have said, there's a chance that your mother didn't tell you the truth because there's something about the circumstances of the conception that she finds traumatic, but unless she tells you, how would you possibly know and therefore how can you be sympathetic? And if she does ever spill the beans on what happened, how would you be able to trust her account after she lied to you for decades? I mean... SIXTY YEARS? That's a hell of a lie, isn't it? You must feel completely blindsided by this.
I also think the fact that your mother is a difficult woman, who isn't kind to you and criticises you and your lifestyle, exacerbates the whole situation. It's easier to forgive someone for something that's a one-off, knowing that the circumstances must have been truly exceptional for them to have done it, than it is to forgive someone who has consistently been difficult/unpleasant/negative towards you.
Only you know how you truly feel, and only you can decide if you want to continue a relationship with your mother, but if that decision was to cut off contact with her, you would not be unreasonable.
All the best, OP, and I hope things go OK for your son re. any tests he needs.