My parents got divorced when I was three. Saw my dad for a day on weekends, if he remembered to show up. That got better over time. I now get on well with both my parents.
It's not easy on the parents, and I can tell you that I really don't remember the nasty parts when I was three. Too many dead brain cells ago...
But I can pass along some viewpoints from the battlefield. Take 'em, and feel free to use 'em or throw 'em out -- whatever makes sense to you.
First, the marriage was between your parents. So's the divorce. You're not part of it, as a reason, cause, or anything else. It's between them. Make sure they keep it between them.
Second, you don't have to choose between your parents. They don't love each other like they did, but that doesn't mean you have to change your own mind. You can still love both of them and want to be with them.
Third, you don't have to choose between them. This sounds like a repeat, but it's not. It took me until I was around 19 to figure out that I didn't want to be a go-between for them. If one had something to say to the other, I told them to ask the other directly, and to avoid using me as a messenger-boy so they didn't have to deal with it. Wish I would have learned this earlier. Heck, I wish I would have listened earlier, that I might have learned it.
Fourth, there's always someone to talk to. You can check at your school or religious institution ('cause they're not all churches!) for a counselor, someone you can just talk to about what's going on. You can also ask your parents for help with finding a counselor as well. Trust me -- it does no good to keep it bottled up inside... :madheck:
Fifth, many kids fall into a trap of "if I had done something different, they'd still be together". That's bull-puckey. They decided. Likewise, there's a second trap of "if I keep this guy/gal away, they might get back together". That's more bull-puckey. Your parents are adults, and will do what they feel is best. If you push back, you run the chance of being branded un-cooperative and worse.
Now, that doesn't mean that if you genuinely dislike the guy, you have to pretend you don't. And it doesn't mean that if something inappropriate is going on, you're supposed to shut up about it. Far from it. If that stuff happens, pass it along.
Just don't be afraid to say, "I know you're dating my mom/dad, but this is a difficult time for me. It's nothing against you personally, but I'm not quite ready for my mom/dad to be dating. And it's her/his choice, not mine, so don't take it personal if I don't jump for joy or anything like that."
A "real" person will be comfortable enough to accept that as a valid position, and to be willing to discuss it with you.
Leastways, that's how I view the world, and I rather like it that way
Good luck to you, and peace throughout the process...
*B*
Been there, done that, still got the T-shirt