Breaking up is hard to do

I'm working on a scene for a book that's a sequel to the YA novel I'm currently querying, and I'm getting stuck. Two characters have to break-up because they're about to go off to college in different states and don't see how they can maintain a serious relationship while apart. They've seen their friends try and fail and don't want to get to a point in their relationship where they're angry with each other or resentful that one or the other is unavailable or too distracted with school, etc.

They both know that a break-up is the best thing for them. Despite loving each other, they do understand that they're not "in love" - like the forever, get married kind of love. But they care very deeply for each other. On the one hand, that makes it easier to accept that they want what's best for each other, and give each other the chance to meet someone great in college and find someone that they could be "in love" with. But at the same time, they really don't want to give up what they have, because it's so special to both of them.

I've been running through possible scenarios for how they break-up, and all I know for sure is that it's a very emotional and difficult decision for the two of them to come to, but they both saw it coming. I can't decide who brings it up, though, so I'm having a hard time sitting down to write the scene. 

... 

Maybe I should just write out the dialogue without assigning lines to either person until I figure out who's saying what.

I think part of my hesitation is that I don't want to split up this couple. They've been really good for each other and supported each other through difficult times. It's been a healthy relationship from the beginning. Sure, they had their ups and downs, but they always worked through things because they cared about each other more than getting their own way. After previous relationships I've put both these characters through, it was a big step, emotionally, for both of them.

But now they need to move on. They have to continue to grow on their own so they can be ready for the next relationships I have planned for them.

I've never personally experienced an amicable break-up. I'm not still friends with anyone I've been involved with in the past. I'm using my imagination to write the emotions each character is going through during the break-up. I wish I had a bit more to go on, but I wouldn't be a writer if I wasn't decent at extrapolating from my limited life experiences to create life experiences for fictional people in fictional situations I can only barely relate to. So, that's my plan for this scene - don't write what I know, write what they know. Because my characters have a tendency to run the story themselves anyway.