My mom passed in 2007 when I was 21. Still lived with my dad and brothers for about 5 years after that, when my dad started dating a woman we knew when we were younger, but hadn't seen in years. 2 weeks after we found out he was dating, he announced they were engaged and were to be married a couple months after. I was completely heartbroken, I didn't know how I should feel about the fact that my parents were married for 25 years and it just kind of felt like he was throwing all that away plus the fact that he kind of turned our living situation upside down (long story). The whole entire time I was trying to deal with this on top of my mounting depression and anxiety, all of my aunts, uncles and cousins told me that I should be happy for him, because his happiness was all that mattered. I never wanted him not to be happy, I just wanted time to get used to the idea (although now that I think about it, I'm not sure how long it would have taken me to do that). I don't mean to scare you, I don't think anyone, including your father knows how this will turn out.

I wish I had some words of wisdom to help you get through this but the best thing I can tell you is you're allowed to feel how you feel, regardless of what those feelings are. And I'm sure you want your father to be happy, but that does not negate your emotions about the whole situation.