Husband cheated 20yrs ago why cant i let it go and forget about this other woman?

my husband cheated on me 20 yrs ago. I never met the other woman and have thought I had gotten over the betrayal but for some unknown reason here lately I just can not get get this woman out of my head. I want to see her face and I want answers...why after all this time do I feel the need to have answers. why cant i just forget it and move on?