What should I do…

Ya’ll I had this letter written to me from an old friend and I never really read it properly until today and I just found out it was a confession…
I feel bad because I guess I was too dense to notice and now I’m wondering if I should give them a response or just let it go because it’s been such a long time. But, for some reason it hasn’t gotten off my mind and now idk what to do
Also we kinda fell off a little bitter bc we were both going through stuff and I felt like we got along rly well but brought out the worst in each other