Complex sentence: He was not angry at her; rather, he felt inadequate and insecure, as if he would never be enough given his lack of experience, but by the time he realized this, it was too late, and she had moved on.
Split into simpler sentences: He was not angry at her; rather, he felt inadequate and insecure, as if he would never be enough given his lack of experience. But by the time he realized this it was too late, and she had moved on.