Heh, welcome to my world. I won't advise you act the way I did, as I am getting the impression you feel differently about this situation than I did. Here is what I do suggest:
One thing you need to remember through this whole process is that it isn't just you who needs to adjust to this new situation. I'm sure your mom feels pretty good right now - free maybe. She's not forgetting about you, she's just using this opportunity and going with it. Which is fine. If going to the country fair with her really means the world to you, then I suggest just talking to her. If she's *not* anything like my mom (which I hope for your sake, she's not like my mother), she'll understand. Just be honest, maybe present it like "Mom, we've always gone to the fair together, and with all the changes recently, I don't really want this to change too." Surely, that'll melt her heart and she'll take you, buy you cotton candy, the whole nine yards. And if you feel comfortable, maybe even suggest to her that you, your mom, and her new boyfriend turn this into a bonding opportunity. I'm sure that it's probably important to her that you like this guy she is choosing to date, right?
If you don't want to suggest that, maybe it's time to move on from this tradition. You're currently living with your mother right? Maybe you and your dad can spend a day at the fair.
Divorce changes everything for everyone. You just need to remember that it's hard for your parents too. I'm sure your dad would jump at the chance to take you to the fair and spend time with you, because what a shock to see your child every day one day, and the next you're seeing them every other weekend.