Let's not kid ourselves. Many of us feel at least a little resentful when we see or hear about the success of others in something we want ourselves. Deep under this sourness is a certain amount of envy, whether it be conscious or subconscious. Envy is a tricky thing: it's culturally shamed, and most people will refuse to admit when they have it. So this envy gets hidden and pushed down deep into ourselves, and at some point it becomes bitterness.
The thing is, it's totally natural to want what other people have. In fact, I would argue that seeing someone with exactly what you desire can be an effective motivator. Just don't let envy get the best of you.
Bitterness leads to making excuses. For example, if you want to be rich, but you subconsciously hold resentment towards rich people, you will tell yourself things like, "Most rich people got their money through privilege, inheritance, or luck." Even if the thought is true, ruminating on it leads to the mindset that you will never be rich yourself. Psychological barriers like these prevent us from seeing our own potential to reach success, whether we have luck or privilege on our side or not. In addition, is it really possible to achieve a goal if you dislike those who have? It's a lot harder to become a rich person if you hold a grudge over rich people.
Instead of disliking someone because they are successful, we should take a step back and ask ourselves to really understand our emotions. After you really think about it, you realize that you are projecting frustration with your own circumstances onto the other person. Think about it: if you feel yourself immediately disliking someone, but you can't exactly say why, it's probably because you envy some aspect of their life and wish you could have it yourself. It's misdirected animosity. It's not a crime to be successful.
I think it's much healthier to be humble and get inspired by the achievements of others, not threatened. If you envy someone's success, wouldn't you rather channel those emotions into inspiration rather than cynicism? We all need to work on changing that unhealthy twinge of annoyance into productive motivation by just saying, "I want that too. What exactly did she do to get it?"
If you really want to achieve your goal (wealth, fitness, fame, whatever it is), you have to let go of every last bit of contempt towards those who already there. You have to open your mind and be willing to engage in the process of getting to that point. You have to be willing to follow in the footsteps of those who have what you want. Most importantly, you should realize that other people achieving something first doesn't mean you won't achieve it at all.
Many people who are successful achieved their goals by getting inspired by others, not threatened by them. They were eager to get down and dirty and learn everything they needed to know, even if they started from the bottom. There's absolutely no shame in looking at someone and thinking that you want something they have. You should be determined to get there and learn from others' victories and failures. Find mentors that can teach you their wisdom. If you have achieved something that you're proud of, be willing to help others achieve it too. Start to foster relationships that benefit both sides and challenge you to be your best self. Let go of the bitterness and start working towards your goal today.