Identify yourself. You’re a bit suspicious of other people’s successes, imagining that they must have had some sort of advantage to be doing well, no matter what their accomplishment. It’s important to you that you’re perceived as being at the top of your game and you don’t particularly like acknowledging that other people can also be high achievers.
You may think you’re getting away with being a closet cynic, but your attitude is probably more transparent that you imagine. But celebrating on behalf of others doesn’t mean you’re conceding that they’re somehow doing better than you are, and if you can feel genuinely pleased for them you’re likely to be happier and more relaxed.
Perhaps you’re setting yourself impossibly high standards? Be a bit kinder to yourself and you might feel a bit more generous towards others.