Ex-husband breaks up and is sad but has a new girlfriend after 4 weeks?

My ex-husband and I separated after 10 years. It all started in August 2024. He wanted to separate because I was hiding money from him, and he found out through surveillance. At the end of the year, he broke up with me because he was afraid I had someone new. He was sad for a few weeks. But after the physical separation, he had someone new just three weeks later. And he told me about it with great joy.
I find it very sad