I guess since it got nuked from mangadex there won't be much in the way of future convos, so my two cents:
I feel bad for Ishihara, but she walked away from her relationship and never made the effort to go back to it. Not because she didn't like Watari or anything, just not badly enough to find a way to make it work. She was limited because she was scared about how her feelings for Watari were changing her into someone she didn't like and she never pressed Watari to get closure with Satsuki, allowing that relationship to be rekindled. It's possible that Ishihara could have won, but it would 100% mean that Satsuki loses Watari and vice-versa. She'd rather they all be friends, and this is the outcome. For Watari's portion, I think when he realized that he and Ishihara didn't even know each other's birthday, that what they had when they were dating was only the most surface-level of puppy-love infatuations. Feeling the shame of that he probably knew then that what they had would never have worked even if allowed to continue.
But yeah, Satsuki stole him, but he was always her's to take from the very beginning - it just took her a while to realize how badly she wanted to fight for it.