Why do I resent other people's success?

Notice that you are not asking to feel better, you are asking to understand. That is the harder and more honest question. Envy is rarely about the other person. It is a signal pointing back at something in you: a want you have not admitted, a fear that you are behind, a place you feel unseen. So when the resentment rises, get curious instead of ashamed. Ask what exactly stings. Is it their money, their ease, the recognition, the freedom? The specific answer tells you what you value and where you feel you are missing it. That is useful information, not a moral failure. Next time you catch the feeling, name the thing underneath it in one sentence: I want that too, and I am afraid I will not get it. Said plainly, it loses much of its grip. Then you can decide what to do about the real want, which is something you can actually work on, unlike someone else's life.