It's okay to try to be the main character but Everytime I end up just like stannis... Is there any grammatical and any other mistakes..

A corrected version:
“It’s okay to try to be the main character, but every time I do, I end up just like Stannis.”
Changes:
  • Every time is two words.
  • Add a comma before but.
  • Stannis should be capitalized.
  • Adding “I do” makes the sentence grammatically complete and smoother.
If you mean acting like a protagonist rather than literally being one, this is more natural:
“It’s okay to try to act like the main character, but every time I do, I end up like Stannis.”

Simplify the sentence

I try to be the main character, but I always end up like Stannis.

Not only I who try to become like the main character but Everytime I do, I end up like stannis.. Is there any need of clarification

Your sentence needs clarification because “Not only I” is incomplete and sounds like you mean other people also try to be the main character.
If you mean you personally keep trying, write:
“I try to be the main character, but every time I do, I end up like Stannis.”
If you mean you are not the only person who tries, write:
“I’m not the only one who tries to be the main character, but every time I do, I end up like Stannis.”
Use every time as two words, and capitalize Stannis.
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