Please Help!! How Do I Tell My Husband He is Not the Father of My Son?

I am so ashamed of this and it has made my life unbearable for me. I’m very suicidal and I just don’t know how to fix this mess. I have a three-year-old son now, when I got pregnant I thought my son was for my husband and not my then boyfriend. I was proud to be pregnant and never had worries about who the father of my baby was.

My husband used to cheat on me a lot and I cheated on him and so when I got pregnant I was sure the baby was his cos I dumped the other guy and later heard that he has passed away. However, problem started recently when I found out my husband has a daughter who was born 7 months after my son and the girl just resembles my husband like a photocopy and I started seeing difference in my son who suddenly looks nothing like my husband.
I see that my son could be the other guy’s. All these while, I didn’t think for once that my husband is not the father

My husband and my son have a very strong bond, they are so in love with each other. But, I fear what could happen if he finds out he is not actually his son.
I feel like a thief. I feel like I’ve committed the biggest sin in the world. My husband’s family love my son, he is one of them they also do everything for him.
Everyday, I want to tell my husband about this but I’m scared I’m always in tears. I’m so scared of the damage this issue is going to do to us. I know his heart is going to break if he finds out. What should I do? Should I tell him or just forget about it? What will happen if he finds out himself?