I’m totally with @seazen in his response.
You’re the only one that you can or should attempt to control. If you’d stop being jealous – let’s assume that’s possible – then you could be a lot more calm and patient in whatever she does. And if she lets you down and cheats with him, then you can take appropriate action, whatever you deem that to be: ignore it, break up with her, cry, or whatever suits you, short of verbally or physically abusing her.
People often live up (or down) to our expectations. If you simply expect that she’ll keep her word (even though she hasn’t always), then she might do that. If you expect her to cheat because she’s spending time with him, then she just might do that, too. Let her know that if she chooses him again, then that’s it between you, and she can go her own way with your good wishes. Otherwise, simply expect her to be faithful. In any case, you need to stop talking and worrying about it.
Worrying about it, being angry about it, “being firm” about it (whatever that means) and talking about it endlessly… what has that gotten you? You can’t control it, and obsessing about it doesn’t help either of you, or the relationship, and she’ll do what she wants to do anyway.