Well, I'm a 24F who JUST got dumped by someone very like your ex's new guy/your ex. I identify with you. Listen, you're not meant to be with this girl. From the sounds of it, you want very different things from your lives. That's what I keep telling myself.
I was dating this 26M for nearly 5 years, albeit on and off. He's a hilarious life-of-the-party kind of guy. Big and not fit, but handsome. He dropped out of university when we were dating. My family disapproved. He lives in an apartment funded by his parents, and only this week, at 26 years old started a full time job. Previously he was employed at very cool seasonal jobs which allowed him to travel and manage events for university students. Nothing wrong with this, objectively. He's just so not meant for me.
But I am an extremely hard working gal. During our relationship, I went through two business degrees, a job managing a fitness studio while also instructing. I like nice things (I'm not spoiled or a gold digger, I just appreciate pretty things) and probably most importantly, the idea of security. Basically, even though I loved the idea of him, being with someone so fun-loving and cool, I know we wouldn't ever work.
That said, I can't stop thinking of him. All the what-if's. The worst part is though, he didn't even treat me that well. He did in his special way, but generally wouldn't do activities with me, my friends or family. He even cheated on me last year while away for work. No sweet clue why I hung around, but I did. And then a miserable year later, I got dumped. OOPS that turned into a personal rant...
Point is you guys sound like you are hella different people. It'll hurt to see ex's with someone else. You'll compare yourself (I find ambitious people do this a lot- myself included), and it'll be confusing and suck. Because you guys aren't meant for each other. Of course this will be someone very different from you. That's ok. I know my ex will find someone faster than I will. He's got friends for days (where I have 3 or 4 best friends) and he's got plenty of ladies after him.
But you know what? That's ok, he'll find a girl that suits him. I'll be very jealous. She'll be very different from me. No better, or worse. Just different. And I deserve better anyway. So do you. I'm not saying she's a bad person, but I knew 3.5 years in that something was wonky. That we weren't on a good path. But I fought harder for it. And that's not a regret, but it is something I hope to learn from. trust your gut. You knew it wasn't working. Don't let an unrelated 3rd party make you question yourself/decisions into getting back together with someone whom you're not meant for.