I know how you feel. I have questioned the paternity of my son since he was born. I suspected cheating which was confirmed Dday two years later.
This is just for me!!!! I had to look at my motivation. Was my motivation in the best interest of the child? Was my motivation in the best interest of myself? My real motivation was that I wanted to prove that my fWS was lying to me. Then what?
Again, just for me. I have found peace in being the best dad in the world to my son whether he is mine or not.