How do you guys forgive yourselves?

Quite often i get in a really negative headspace and my brain just floods with overwhelming thoughts of every shitty thing ive ever done throughout my life. I already have a low self worth and this just adds to the crippling pain. I know im a good person but ive done some real fucked up shit and i just cant bring myself to forgive myself. How do i do it? And how do i stop doing shitty things?