I'm a very forgiving person. It means I do not always win, and sometimes I get taken advantage of, but being able to forgive & forget means that I can put things behind me and move on with my life without bitterness eating away my insides.
We cannot change the past, so why continue living in it? Sometimes we need to move on (as hard as it may seem to do).
Forgive & forget does not mean literally wiping what happened from your mind. It is about acknowledging that you can't change what happened, and getting on with life without the memories of what occurred holding you back.
I class being unable to forgive or forget as a form of bitterness, and we all know what they say about bitterness. That it is like taking poison & hoping the other person will die, or that it is a cancer that eats the host from the inside.
If you cannot forgive or forgot, then you are also denying the other person the opportunity to make things right again.
Of course, some people are just bad for you & should be cut out of your life, but I've found that, in the main, most people who do the wrong thing feel embarrassed & ashamed that they have let the other person down & want to right the wrong. That requires forgiveness from the wronged person.