resent towards my father :(

I’m 18 and my whole life my mom has been a mother/father figure to my little sister(16) and I . My father has never been interested in us only when he was forced to by my grandma & his many girlfriends or when he needed money when we were younger . Since we became of age of where we noticed things , he’ll call on us to “check” on us and usually calls only when he needs help with something or when he needs money .

At the moment, he has a girlfriend and she has a daughter . Of course I want him to be happy and all but it still makes me sad/angry that he’s posting them, saying he loves them and making promises to them that with my sister and I , he’s never completed ! He’s promising that he’ll give them everything they need when with his own daughters he won’t take out to eat ( and that’s something simple )

His girlfriends daughter calls him dad and she’s only 7 . Like what kind of woman will tell her child to call a man ‘ dad ’ when you don’t even know the guy and how he treats his biological daughters . And his girlfriend has never spoken to my sister or I in the few months they have been “dating.” And the one time my dad told her to speak to me because I was in the car , she acted like if I wasn’t there .

I just feel like I resent him and I don’t have no one to talk too because every friend of mine has grown up with their mother and father in the same household .

Don’t get me wrong I have a wonderful stepdad that pretty much raised me since I was 4 and my sister since she was 2 . But I still have a void in my heart because I don’t like feeling this way .

To add more to the story , my dad has always put his girlfriends and his girlfriends kids above my sister and I . And he’s never given us money for school or clothes and he always make promises he can’t keep and it’s the same thing every year .