Ex sent me a long message asking for another chance… then I found out he was hanging out with the girl he cheated on me with

I feel sick and I need outside perspective because I genuinely don’t know if I’m overreacting or if this is as bad as it feels.

Earlier this year, my ex emotionally cheated / developed feelings for another girl while we were together. He had wandering eyes, didn’t properly distance himself from her, and it completely broke my trust. We broke up, but the aftermath has been a mess of mixed signals.

Since the breakup, he’s gone from: telling me I liked him more than he liked me, saying he’d never want me back, asking if he might have a chance in the future, saying the breakup was for the best and that I should move on, still doing caring things for me after the breakup, and now recently acting reflective, remorseful, and emotionally open again

A few days ago he sent me a very long message saying he misses me, regrets taking me for granted, has spent the last couple of months reflecting, and wants another chance. He basically said he still hopes there’s a place in my heart for us, wants to grow old with me, and asked if we could slowly revisit the relationship / start fresh.

Since sending that message, he’s also been texting me asking how I am and asking to meet up for a chance to talk about getting to know each other again with the intention of getting back together. Before I could even reply, I found out that he was at a café hanging out with the same girl he cheated on me with. I honestly feel like I’ve been punched in the stomach.

I just don’t understand how you can send someone a huge “I want you back / I’ve changed / I miss you so much” message and then casually be spending time with the exact person you blew the relationship up over.
I know technically he’s single and can do what he wants, but am I crazy for feeling like if he was actually serious about rebuilding trust, the bare minimum would be to not keep hanging out with her?

Right now I feel embarrassed for even taking his message seriously. What do I do?