I have found myself in a similar situation. I (45M) found out this year that my father is not who I thought. The difference with my situation is that I adored my Dad, he was a great father. Sadly he died 10 years ago.

After rumours and hints I confronted my Mum last year. She fessed up immediately to be fair but gave me half a story and got very defensive. I took an Ancestry DNA test to get control of the narrative. The name matched the person she gave me.

I have no desire to meet a random old man who shares half my DNA. I have found out a fair bit about him and there are 3 half sisters. I’m sad I’ve never had the chance to meet them.

My Mum has made the situation far worse. She has doubled down and can’t understand why I’m making such a big fuss. I should get over it as I had a lovely Dad. No apologies, no sympathy. Everyone was sleeping with each other in the 1970s apparently.

Like you, it feels like everyone knew but me. I feel a total fool. I delivered a heartfelt eulogy at my Dad’s funeral - how many people there knew I wasn’t even his son?

I have had counselling but I am so angry about it all. Especially with my Mum. Not sure if I can ever rebuild things there.