When my mother remarried I was very unhappy. At the time I was about 10.
I made life miserable for my mom and step-dad for years.
I feel so bad about that now. My mom had been alone for years and she had finally found a man who made her happy and I was a nasty kid. I was a lot younger then you though and acted like a child.
Now my step-dad and I are the best of friends. I homeschool my kids and we spend at least one day a week with Grandpa. It's a blast. The man is awesome, I just wish I'd realized it back then.
I know it can be tough to see the one you love start to love someone else. However, your dad doesn't love you any less just because he's found someone new to share his life with. We are made with the capacity to love more then one person - thank heavens as I have 3 kids and a husband.
Try to put aside your feelings of what you think he needs and be happy for him. Being alone as an adult isn't a lot of fun.