Sometimes the person we need to forgive is OURSELVES!
I used to hold myself to such a high standard that it meant I was doing almost everything wrong! Could not let myself catch a break. Yea, that did not go so well; I
had to change my internal dialogue and it began with forgiving myself for things I did or said, mainly when I was younger or didn't know better. I forgave myself for not knowing enough about a situation to make sensible choices. I forgave myself for being blinded by what I wanted instead of what was right.
The first step is acknowledging responsibility and this might take on multiple facets, even owning up to an expectation, and look at why it bothers you. Sometimes I look at what I did or said when I was 12! Now I know I should have acted in a different manner... but now I have perspective. Now I am distant from the person I was twenty some years ago, 10 years ago, 4 years ago. Therefore, I chose to forgive myself for not knowing or understanding what I know now; then I give gratitude for how it has helped me grow. Sometimes we have to have those lessons, especially if it gave us negative consequences. (And sometimes we have those lessons a half dozen times.)
If you still ponder on past mistakes whether you hold yourself accountable for being the victim or you know you hurt someone, the time is to gently forgive yourself. Allow yourself the ability to make mistakes, it is ok!
There are professional therapists that can help people with more complex situations and I encourage finding a psychoanalyst or psychologist in your area that can guide you through something like that.