You sound like you lack self-esteem. My heart goes out to you. I sometimes have the same experience. Self-esteem has been compared to a bucket of water. It starts out full when we're born, but whenever we develop negative beliefs about ourselves, it's like poking little holes in that bucket and our self-esteem drips out.*
Stop comparing yourself to everyone, we are all different in our own ways.
Make a list of the stuff you're good at. Give yourself three compliments every day. Don't just say, "I'm so great." Be specific about something good about yourself, like, "I was a good friend to Greg today" or "I did better on that test than I thought I would." While you're at it, before you go to bed every night, list three things in your day that really made you happy.
Also about the grades, don't worry too much about them, most people don't even have one pass in their GCSEs. When you hear negative comments in your head, tell yourself to stop. When you do this, you take the power away from the voice inside that discourages you. Think positive thoughts about yourself! Focus on your strengths – not your weaknesses. Realize that you are better at some things than others.
Most people who are good at sports etc were not born with it, they developed it, maybe genes helped here and there, but try and develop something for yourself too. I am sure you are good at something, you just have to discover it.