I'm sorry you are so hurt OP but can I explain my experience to maybe give you a different perspective.
I'm in my mid 60s and have only ever had 2 or 3 close friends in my life but have a very good female friend (best friend) less than half my age, we'll call her Kerry. I also have sons and daughters with my youngest child being slightly older than Kerry.
We met a few years ago through work and just clicked. She makes me laugh, we share lots of interests and are on the same wavelength about so many things. In return I also make her laugh and I think she values having someone older to turn to when she has ups and downs.
I 100% trust her and know I could rely on her in an emergency and I'm sure she would say the same.
Because of the age difference she also calls me 'Mum' and has sent me Mothers Day cards (her mother died when she was a child)
My relationship with Kerry is different to the relationship I have with my children and knowing Kerry doesn't mean I love my own family any less but I do seem to message and chat to Kerry more than I do my own children. Also like your father I would absolutely want to include her in any inheritance although I haven't got round to changing anything yet.
Unlike your father, I am aware that my children could find my friendship with Kerry threatening so I don't go out of my way to relay much about Kerry back to them although from the little they know they think it's strange that I get on so well with someone so young.
In my case geography means my children are unlikely to meet Kerry and as she is very different to my own kids I'm sure she would find them intimidating if she ever did bump into them. In return they would probably find her a little 'rough around the edges' (that sounds an awful expression but I can't think of a better way of putting it) and would not understand why we are so close.
I really value my friendship with Kerry and would be heartbroken if I ever had to chose between my children and Kerry. She definitely adds something to my life that I don't get from my family in much the same way any close friendship would.
In my case no one is abusing or manipulating anyone and neither Kerry or I have any sinister motives for maintaining the relationship other than we both find it beneficial, can chat for hours and enjoy each others company. Could your father's relationship with Tracy be similar?