Speculating about the next guy (or other guy if it's cheating) is mental masturbation and narcissistic thinking. It's a waste of time. I've been there. "Oh, she just wanted a pushover," or "she wanted to ride the c0ck carousel" or whatever. This kind of thinking always flows back to you. It supposes that you and the other guy are zero-sum 2-dimensional dudes and that she lacks agency in her decisions. More importantly, it feeds the ego and pushes against personal growth and change. That is, if you weren't serving up what she ordered and he was, you can feel like you weren't the problem or that there's no need to learn and improve after this.You are right, I think she got a dude that is esy avalible and willing to commit but of course I'm not sure.
Do your best to bulldoze all thoughts of her, him, and anything else out of your brain, but not before taking stock in how you would handle such situations differently. As an example, in the past for me it's been a reluctance to disappoint people or to be honest about my goals and desires. I'm a more brutally honest person now and it saves a lot of long term discomfort.
NOTE that I'm suggesting you learn and grow, not beat yourself up over what you could have done - the past is the past.