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Well first thing, you're 13, stop lifting weights!!! You're body is not ready to lift until you are 16-18. Trust me i've seen enough 18 year old with spinal problems to tell you to stop. Second, it's the first time youre doing it, did you expect to ride a bike the first time you got on. Push ups and bench are the same muscle groups, but not quite similar on performance. Do you do push ups and bench to where the floor/bar touched your chest. Balance when starting is a lot easier for push ups, anyway, i could go into more detail, but for now at your age, push ups, crunches, pull ups, sports, body weight stuff. Get a little older before you get hurt.

there is nothing wrong. you develop muscles specifically to do certain tasks. If you cannot bench as much as your peers, it is because this motion is new to your body. practice and over several weeks time, your stabilizer muscles will become stronger and allow your major muscle groups to release all their energy on the weight instead of trying to balance the bar