Ex is a narc and now has new gf in office - help!

Unfortunately I got involved with a guy with a history of cheating on previous girlfriends. The last ex he cheated on by having an affair with someone who is married and works in the same office for nearly 2 years. I ended our thing quite soon after I learned what he had done in previous relationships but the office situation is excruciating because his ex-lover is still there and they have been flirty with each other and even arrived together in the morning (that was so painful to see).
Now I have learned that he is seeing a girl in a different office (same company) and she is a girl who almost every guy I work with lusts after.
I can't seem to get over the situation with him (he chased me immensely after I ended it claiming that he really wanted just me and wanted me to give him a chance - despote some very questionable behaviour with his ex-lover).
I definitely don't want him as I had multiple opportunities to be with him but I can't bear the thought of him being with this new girl and treating her better than me. I am not a patch on her (looks-wise) so I'm assuming that he won't cheat on her or treat her badly because she is such a great catch (she's actually really nice too).
I need some advice and help to get through this horrible situation. I realise I still have feelings for him but they are ones that I would absolutely never act on. I just want to move on but his behaviour in the office (with ex-lover, new lover and he's pretty loud too) is making it so difficult...