Disappointed in my ex of two years

For context on why my ex is still relevant though we've broken up - we still live together and will continue to for the foreseeable future. So unfortunately, our lives are still very much intertwined.

I broke up with my boyfriend of nearly two years on 4/13. We had been having a rough go of things for a while, and breaking up had been in discussion prior, so I thought it was fairly mutual. Neither one of us was happy. What ultimately led to our break-up can be boiled down to conflicting personal values and wildly different perspectives on life.

Before getting together, my ex and I were coworkers. At the time I started the job, he was dating a woman that worked in the same organization, but in a different department. I interacted with them daily, and was witness to their relationship fall-out spurned by the woman's infidelity. Several months after their breakup, I started seeing him. However, his ex was still involved for a little while because she had belongings at his house that she was unwilling to collect. She eventually did, but she was disrespectful of both him and me in the process.

Months later, I found out my ex had texted with her on several occasions, never mentioning it to me. Their conversations weren't suggestive in any way, but I felt betrayed since he kept it a secret, and confused since she had treated both of us so badly. He didn't see anything wrong with his behavior, but he stopped talking to her after that.

All of this preface to say, I found out today that he started talking to her again on 4/15, two days after our breakup. Though I admit he has every right to speak to whomever he pleases now, and I don't regret my decision in ending our relationship, I can't help but feel wounded by it. I'm also confused as to why he would reach out to a person who had inflicted such harm on him, and who is also currently in a relationship. Of course, she has reciprocated his messages since she has no problem with skirting around relationship boundaries.

Beyond (flirty) messaging, they've also been getting lunch several days a week and going for walks on their breaks at work. I feel sick about it. It just feels like I only got to know who my partner was recently, just before and after our breakup. Now that we're no longer "obligated" to each other, his real character has fully come out. And besides this behavior, he's been snide and rude to me. I'm grateful he's not a violent person, so I'm not worrying about my safety, but it's so uncomfortable to be at home at the same time as him now. I can only hope I get out of this environment soon.