Is it immature to hold a grudge against a girl that dumped you for a richer and older dude?

The girl I dated casually for a month recently dumped me for an older and richer guy. When we began dating, everything was great but during the last two weeks before I was dumped she lost interest in me. I tried to make it work but it just did not work out.
One night she told me we need to talk. She told me she is looking for a husband and someone to settle with. She claimed I was great but too young for her and that is why it is unfair to continue such a relationship. She added that she is going to be actively looking for a man that is much older than her, wants a family and has a stable job. Fair enough. She concluded with words that when is actively looking, she finds guys very quickly. At that moment I thought she wants to get married ASAP, first come, first serve type of deal. And I was almost right. I felt bad for her. After she dumped me, it did not take a week for her to get together with another guy. That guy is 9-10 years older than I am, a somewhat famous DJ and a CEO of an apartment complex. I can't help but to think that she is a gold digger. All of her previous boyfriends were rich foreign men and this one was not an exception.
In the end I just felt bad for myself that I was dumped and now I can't find a girl. Been dating a few but nothing serious.

Weeks have gone past but I still hold a grudge against her. Am I being immature? The break up itself does not hurt me as much as the fact that she got together with a rich dude so quickly and that she was probably talking to him while we were dating. :( I feel used and worthless. I see her quite often at school and I act neutrally towards her, just greet her and that is it. Sometimes I just want to call her out on being a gold digger but that would be immature and just make things worse.
Some people say I dodged a bullet so I should be happy she is not my problem anymore. What do you think, people? Any advice
Is it immature to hold a grudge against a girl that dumped you for a richer and older dude?
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