There's Hope For Hopeless, But I Still Haven't Found My Happy Ending Yet

The sun is shining. Spring is finally kicking winter to the curb. My friends and me are all on great terms. My family and me are on great terms right now. I'm doing well in school once more. I'm a bit stressed, but honestly, I have no complaints about my life.
My life is going so well right now. It really is.
But guilt is a monster that is slowly eating me away, willing me back into the darkness. I can't enjoy life when I know I don't deserve to.
I made a mistake a while back. A really, really big, and really, REALLY bad mistake. Something that if anyone knew they'd hate me forever. I'm not exaggerating, it was that bad. I've had to keep this inside for so long because I can't tell the truth, I'll lose everything if anyone finds out. But I'm losing myself because of my stupid decision.
I want to enjoy my life and be truly happy, but how am I supposed to do that carrying this shame around? This is something I'll never forgive myself for.