Feel betrayed by his lack of trust

[60goingon30]

My partner and I have been together for 8 years. He has a 17 year old daughter whom he's brought up alone since her mother died when she was two. My children are older (almost independent?!) So I've basically mothered her since she was 9 and her autism hasn't made that an easy task.
I found out, by chance, that my partner isn't actually her biological father. I'm bombshelled that he hasn't trusted me enough to tell me this himself. His grown-up children and other family members know this and I feel I have been excluded from this 'circle of trust.' I obviously don't feel any differently about his (very vulnerable) daughter. It's the lack of trust and his feeling that 'it's none of my business.'
I'm struggling to get to grips with this and he's made me promise not to talk to anyone about it, so I'm quietly going mad. I need to be able to discuss my feelings.

maybe as far as he is concerned the 'biological' fact was irrelevant to him? as far as he is concerned, she is his daughter! sounds like they have already been through a lot of trauma, why add too it. he's stepped up, she's his.. end of discussion.

perhaps he was worried that this little nugget of fact might have affected your relationship with her?

Thing is, it's not really any of your business.

It doesn't affect the relationship between him and her nor you and her.

It's not that you were kept out of the circle of trust but that you had no need to know.

I can understand why you feel like this but logically, it's not really relevant.

I can see where you’re coming from if other family members know and you’re in a long term relationship. Is it that it’s irrelevant to him and after the trauma of losing her mother he doesn’t want to revisit this? It might be painful and he just didn’t want to face it and have the conversation. To all intents and purposes he feels like her father so didn’t feel the need to bring up the technicalities ?