It doesn't sound to me that you dislike your father's girlfriend.
It's too much to expect a child to go from supposedly being traumatized by a parent's affair lover to being open, loving, and accepting of a new lover at the mere stroke of a pen on a divorce paper.
And it's not uncommon for dad's new girlfriend to be younger than his mom, and perhaps not many years older than the teen boy.
Lots of teen boys in that situation can feel a whole range of jealousy Sad Teen did not mention.
I wouldn't be surprised if you are angry and jealous not just at your father's girlfriend but also toward her son. Second, the divorced parent is often so happy to be in a new relationship that s/he begins to spend less alone time with their children.
There is a lot that I need to tell you about the feelings of children when their divorced parents begin to date. They are often so consumed by the new relationship that they are unaware of how their behavior is affecting their kids.