When your ex is seeing or dating someone else it is not just harder to get back together, it is also a lot more uncertain.
There is always the chance that your ex could decide that they want to be with you because you are who they love and the one who makes them the happiest. But your ex could decide that even although they are happier with you, the relationship has too many problems and they can’t see how things could be better.
They may not be sure yet if they love the new man or woman, but the new relationship does not have the same problems and therefore the new man or woman is who is good for them.
Your chances depend on how you can demonstrate that things could be better.