Forgiving someone

How do you honestly forgive someone & forget? My current bf & i have been through a lot.. he promised to change & he has. Things have been great with us recently but his past of what he’s done to me haunts me.. sometimes i catch myself thinking about it all the time & getting so angry and anxious. Yesterday i kicked him out for something he did a YEAR AGO!! we’re going to counseling and the counselor said i need to let go & move on if i want it to work but it seriously haunts me :( how can i let go of this & not let it control me anymore?