Why did he let it happen??

as a little girl i used to idolise my dad, he was my hero. when someone asked me who i wanted to marry i said 'my daddy', in my eyes he was the only one who kept me safe. but he never lived up to this. and he shattered my illusions. when i was 4 i recieved a christmas card, i opened it to see a picture of my dad kissing another woman. Then when i was 8 the phone rang and i answered to hear my dad having with another woman. At 10 he gave me his old laptop and on it i found naked pictures of his other women and emails. when i was 11 my dad looked after me and i fell and split my lip, i told my dad i might need to go to hospital and he was so high on coke he just rolled over and ignored me. he left as he has got a new 28 year old girlfirend and she was pregnant. he was my hero, but he never knew that i knew these things. i get so angry at him but i never once spoke to him about any of this ever, wheni see him i act like its all fine and i annoy myself. i cant even write him a letter.