If you're able to forget bad memories it can take a few years and sometimes as long as 10 years to forget a bad memory that happened.
What you need to do is create more happy memories so that your brain gets filled with those happier memories and pushes those bad memories aside or even pushes those bad memories out.
Some bad memories I've had It took me 20 years to forget fully and some of those bad memories stayed with me even now.
So some memories may be so bad that you may not be able to forget them at all and the bad memories may be with you for life.
But some memories you might forget after a few years of creating new better and happier memories and sometimes it can take over 10 years to get those bad memories to go away.