How to get over him when we work together?

I was once in a similar situation. It stinks, and I never got involved with a coworker again! The best way out, both for your personal sanity and professionally, is setting strict boundaries and sticking to them. Tell him that you don’t want any fallout to affect your careers, and that you need space for a while so please respect that. And clarify that means, no flirting, no checking in, no small talk, nothing outside of necessary professional contact.

Then YOU need to make sure your words and actions are aligned and stick with it. If you two were still in a situationship after a year and a half, I suspect boundaries haven’t been strong through this point. So it’ll be a change, and it’ll still suck to see him every day for a while, but it’ll improve after a couple months if you really do limit your contact with him. Plus, if he’s not a bad guy, he probably wants to work towards getting past this and feeling comfortable again at work just as much as you do and will respect it.

Mine ended badly too, though it was much shorter, and I was really, really upset at first and horribly uncomfortable seeing him in the office every day. But with a very brief bit of discussion about each of our boundaries and work expectations (awkward convo, but important for staying professional), and a brief clearing of the air / apology after a few months of space, we were able to get over things and patch them up just enough to resume taking occasional (platonic!) coffee breaks together. So that was a big improvement since it meant we were no longer uncomfortable, but the time with limited contact was necessary to get there.

I eventually got a new job for reasons unrelated to him. And we didn’t really speak again once we were no longer coworkers who had to see each other every day.