With respect, saying that "she left for someone better as I expected" is such a loaded statement. She left. That much is indisputable. She left for someone better? Your self-esteem is almost certainly getting in the way of a successful relationship. He's not better; he's different perhaps, but certainly not better. The guy might cheat on her three months from now and you've wasted three months telling yourself that he is better than you...? And what if they live happily ever after? Then he may in fact be a better fit FOR HER; but he's STILL not better than you.

But the part of your statement that really troubles me is the whole, "as I expected". Here's the truth, if you believe that the relationship is going to come to an end than it absolutely will - but even if you don't expect the relationship to end it still probably will. Every relationship you are ever in is going to end - except the last one. All you're doing now is gaining experience and skills to take into that relationship, enjoy the ride.