You can tell your friends something brief and simple. Something like you realized that the two of you had very different life goals and ambitions and the two of you couldn’t find a compromise that made you both happy.
I’m late to this post, but I want to reassure you that your (maybe ex?) fiancé will be fine. I speak from experience. My ex-fiancé called off our wedding 2 weeks before, and it was the best decision of my life. At the time, of course, I was pretty pissed, but pretty quickly I realized he was brave for doing so. The warning signs were all there and I didn’t want to see them because I’d planned a wedding. I’m so glad he’s my ex-fiancé instead of my ex-husband. Don’t beat yourself up. She’ll be okay.
Wow, that was brave. I think you did the right thing, because a bunch of random users on an advice site wouldn’t have been able to sway you if you really wanted to marry her. I can’t imagine how hard that was for you to do. I’m sure you’re hurting, and obviously so is she, but this is much better for both of you than a divorce. Especially a divorce with children involved. Good luck going forward.