Why do I feel upset about my dad breaking it off with his gf of 2 years when I didnt feel upset when he?

Maybe you saw a lot of yourself in your father's girlfriend. Not in a creepy way but the personality traits and such. Maybe you have a great relationship with your mom, but you could see they just weren't good together, so you accepted that. And your dad breaking it off with the girlfriend has made you almost feel like he's breaking it off with you or has rejected a person similar to you and therefore rejecting you.