I left my husband (non A) but it sort of evolved rather than a big break. A few points:
- get your finances sorted in advance. Protect yourself without being unfair. If you have shared bills or rent think about how this will be handled in the future.
- who will move out, and the logistics. The cost of moving.
- how you will tell him. Make sure all your ducks are in a row first.
- legal advice; should have put this first.
- family support. Who can you rely on? Who will side with him? His parents as grandparents. Be prepared for them to turn against you or not be supportive. That's just what families do, so expect it.
- be careful about when to tell him. Often the gloves are off once a partner decides to leave. Keep this in mind.
I know I'm not addressing the emotional side, but being in the rough and tumble of a breakup and having to also work out logistics is tough and may cause hasty decisions, which is why I suggest planning.