Working with an 'ex'

I had a brief 4 month 'relationship' with a client that the company I'm employed by does work for. Went out a couple of times, he stayed over in mine 4 times. Would text and call numerous times everyday, very involved in each others day to day lives. From the start I had made it clear I didn't want it to be a casual sex only thing, which he told me it wasn't. We are both very busy with work atm, and getting a chance to meet up or go out was difficult but we were managing it, and he was talking, as I was, about how we would make up for it when things quitened down.
Out of nowhere he texted to call it off, saying he wasn't feeling himself, he had a lot on his plate, didnt want commitment, wanted his own space. It was a very blunt message. Now in fairness, I like my own space too, and the level of texting/calling was getting a bit too much for me as well. I cant understand the complete 360, but I am aware it does happen and no one knows whats going through another persons mind. I told him that was ok, we could take a break whatever, that I was there if he needed to talk etc. And he never responded. I'm considered a very good natured person by most people, and I find it hard to walk away from someone I care about when they are suffering in some way, which he clearly is.
However, the manner in which he called it off hurt me. It made me feel used, and worthless if Im honest. I'm not someone he met briefly online or out, who he'd never see again. He knew that he would have to deal with me on a work basis.
This is my problem.... I've had to deal with him professionally three times already in the past week and a half since he called it off, and while I'm very proud of the fact I haven't had any sort of meltdown in front of him, remained calm and professional, and got on with my job, I'm not sure how I can keep this up. Its like dealing with a complete stranger, not someone who rang me up to 3-4 times a day up until two weeks ago.He is very cold, distant and makes me feel terrible. I realise that the problem is with me really, if I wasnt so sensitive I could rise above it. I'm not annoyed or angry about the thing ending, it always had an expiry date, more the way it was done, but how to get past this and get on with working together? Theres no way out of meeting him through work, unless I tell my boss the scenario which would be incredibly awkward and unneccessary because the situation cant be avoided all the time.
Any advice greatly appreciated!!