I don't like my dad's new relationship (Update/new problem)

If you have been following my over dramatic posts here about my dad's new gf then this is a new installment. My dad told me him and his gf are going on a vacation for a week together to Vegas. A few weeks ago I originally said I was fine with it but now I'm not so sure. After meeting her kids and having a spiraling breakdown I feel rather conflicted. On one had I want him to be happy but on the other hand im gonna miss him like shit. After my mom died a year ago I developed a strong kind of attachment/separation anxiety for my dad. And when he was in the hospital for a week. For a while he was the only thing keeping me going. I know he's not replacing me but I don't think I can stand a week without him. I just don't think I can pretend to like this new installment to my life. I thought I had longer. I just now started grieving my mom after months of feeling mad at her after she died. I thought I had longer until he started dating again. But not even a full year after she died. I just I don't know. I feel like a horrible daughter but he's literally my whole life. I just don't know what to do anymore.