Ex-Boyfriend's New Hookup Is My Student

So I'm this high school teacher who recently broke up with my boyfriend of two years. It wasn't a sudden breakup or anything dramatic, just that we grew apart and realized it was time to move on. But, turns out he didn't wait long before finding someone new. And that's where things get... interesting. He started dating this girl who's only in her early twenties, I think she’s around 23 or so. The twist? She used to be my student when I was a substitute teacher at the high school two years ago. It wasn't exactly a secret affair, she'd already graduated by then and came back for college classes, but it still felt a bit... complicated. I ran into them together once, holding hands on campus. My ex had this smug grin on his face that just screamed 'look what I got!'. And me? I couldn't even muster the energy to feel jealous anymore. Instead, all I could think was how weird it felt seeing my old student like that. But here's the thing: she’s a good kid. Smart, kind, she wouldn't have been caught cheating in high school or anything if that's what you're thinking. The relationship seems consensual and mature from the outside. Plus, I've moved on too much to hold any grudges towards either of them. Still, it lingers in my mind every time I see them together. Partly because it’s a bit ironic, I taught her algebra and now she's sleeping with my ex. And partly because there's this niggling voice inside me that wonders what might have been if... but no use thinking about that. So here we are, six months after the breakup, and everything feels like it's moving on just fine except for these weird moments where I'm reminded of something old mixed with new. Guess that's life though, full of unexpected twists even when you think you've settled into a routine.