Just a few thoughts..........
First I am the aunt of the most beautiful girl in the world
, whose life was shattered at the age of nine when her father walked out to be with someone else. I and others in the family have been a big part of trying to repair the damage that was done, and we now have a psychologically healthy 15 year-old, but we still keep our fingers crossed that she is as OK as she seems. Her father didn't walk out on her
(although it seemed so to her at the time) - they have a close relationship that he has been very diligent to maintain, but it did a huge amount of damage.
Second I am very close to a man who did exactly what your father did - had another child while he was married with children. I don't condone at all what he did, please understand that. But he is a very loving father who (twenty years later) has a very good relationship with all his children. He DID try to integrate the 'affair' child with his 'marriage' children, and it has worked out relatively well (I can't speak to how his 'marriage' children felt at the time of course, but they were at least five years older than you were so may have been able to accept the situation a little more readily).
I think your father is hugely underestimating the impact on you of what he did. From what I know, an eight year-old is wired to need the security of a stable parental environment. I think his remark about you wanting to be in the limelight shows that he does not understand the impact his actions had. BUT, I also think his intentions in telling you were purely good. In the case of my friend who did this, he was very concerned that his 'affair child' was in an unstable environment (the mother became pregnant to try to force him into leaving his wife), and he felt that all his children had a right to a relationship with each other. Mostly they have a very positive relationship with each other.
I wonder if it would be helpful to seek some qualified psychiatric advice (I don't mean you need help of that sort!) about exactly what the impact of this would be on an eight year-old? I am betting it is a lot more than your father thinks. And it might help you to understand, at the age you are now, just what happened to that little girl. If you could share that with your father, he might come to understand your feelings.