Most Helpful Guy
Start with I love you, but it's just not working out.
Come into this knowing. Make like, a hundred lists; pro/cons, if/thens, points and rebuttals, etc. Most breakups fail because they get talked out of it, because they have feelings. Don't want to hurt them, do wish it would work, etc.
At this point reiterate it's just not gonna work out
feel free to establish that you care for them, but don't go into detail too much. Accept that you are going to hurt them. Then, dont try to hurt them, at all, but don't actively avoid it either. This is about what you need.
Try to make it about you, but don't be afraid to say why it's also about them.
I have these needs, they aren't getting met, and I don't blame you. I have these wants, they aren't getting met, and I accept that you're not the one to meet them. You have these needs, I can't meet them, or I worry that I can't long term. You have these wants, but I have these wants, and hey just don't match.
When you part, figure all the important stuff out right away. I'm gonna pick up my things at such and such a time. So and so will be with me. You can have my such and such, but I am keeping this and that. Get everything in line before you part, so there aren't reasons you need to communicate overly. The reason for that is, once you have everything settled, make sure to do this within the week, you can say okay. I can't talk to you for a while. Months. Don't say it out loud, but inside your head, you aren't going to talk to this person for three months this.
Then, dont talk to them. Time heals, and if you wanna be friends, after 2-3 months of complete silence, that's up to you.
Most Helpful Girl
I don't understand why you wouldn't clarify your future goals morals and hobbies before you got engaged let alone before having children. Seriously a very selfish decision. Sadly it's already done. The ones who will suffer will be you children. Try to let him down respectfully and explain why. Hopefully you can co parent well for you children's sake.