I want to call my SO selfish
How is your SO being selfish? Can you talk to them about it?
Because he can't get along with my family.
It is good to stand up for your self in a relationship. You can be kind when you confront them so they don't feel attacked.
I always want to win, who cares about making her feel good.
You should be concerned for their feelings because they're your partner. Even if you feel like you're being wrong you should try to work things out in a calm manner that won't leave you both feeling upset.