Why is it always the bad memories that hang around in your head?

It’s always the worst memories the stupid ones, the ones that give you such a bad feeling that you want to just carve away at yourself and the thoughts go round and round and always seem to be there when you’re laying in bed in the shower on the bus at any moment that you get quiet time it always recures and most of the time it’s the same thought again and again then when it becomes an accepted thought there is always another to take it’s place, it’s messed up why? I sit at school thinking about strange things all the time and can’t stop, then people remind me then i have more than one in my head i wish i could stop but i just think all the time i feel depressed but i don’t have clinical depression i haven’t talked to anyone and i don’t plan to because all i’ll do is deny and lie everything i’m pretty sure i’m some kind of scitzo cause i always argue with my own thoughts and i know that i’m doing it i say one thing then think another and then think again to contridict that then again and again till i have a rolling conversation in my head i’ve given up on caring about myself now i seriously am a stuck up shit that no one likes or could ever like suicide is still a recuring thought i just hate myself. Life isn’t worth living but i guess i’ll have to try so i don’t upset my family.