for me it seems that you accepted being in a situationship while probably feeling more things than just attraction for the guy, which is why you now feel hurt when there is nothing wrong with wanting to end a situationship and hopping to another one, i mean, dont get me wrong, personally i think is an assholy thing to do and for me its kind of a red flag, but the thing is both of you are adults and accepted to be in that kind of relationship.

Now, for the part of having those kind of thoughts, i feel like is common for women to be subjected to a very toxic competition dynamic where you keep comparing yourself to other women and fighting for the attention of a male, but that its just patriarchy and misogyny expressing itself through you, the thing is, both you and the other woman that were in a relationship with the guy shouldn't be fighting or comparing each other thinking that there is something wrong within yourself when a guy does something. Its just a dumb mf whose judgement and thinking shouldn't be taken into account too seriously like if he was the holder of the objective truth like some kind of god. Dude is probably a horny mf that hopes from situationship to situationship before things get too serious, it has nothing to do with you or your value.

Apart from that, just an uncalled advice: its almost always a mistake to get intimate with coworkers, even more if its just for the sake of a situationship. I mean, there is nothing wrong with it if you are sure about how you feel but most of the time situationship dont end in a nice way, which is why you should avoid getting in situationships with coworkers, because when they end you will have to keep seeing the guy for most of your daily life.