Why do i always feel guilty?

I keep gettin these guilty thoughts inside my head.I feel like i dont deserve any success or i dont deserve what i acheive.I dont kow why but i keep feeling that people wont accept me if i do my best.I am more the quiet and shy kind of person and mostly people dont realise my talent till they see my work or my grades and then they kind of get surprised .I dont like to brag because i feel guilty about it and so i always point out my wrong s and weaknesses.But i try to cover these feelings by anger and get easily irritated .i am beginning to get these thoughts more and more and am starting to sick of it.I also keep ysing my thoughts.I was brought always being compared to my brother and how he does things better but now i have realised who i am really afraid to show or express it .