Well, I discovered after my husband died that during the last hard month of his life, he had been watching porn on the Internet in the night hours when I thought he had finally managed a little sleep,
Knowing as I did, that he had complained of an inability to sustain or latterly even have an erection and that he had worried about this both for my sake and his, I realised that my reassurance that this would pass (neither of us realised how little time was left to us, at that point) and everything would be all right again, hadn't helped him. I choose to believe he went along with it when I said it for my sake.
Perhaps your husband was in the same dark place?
If it was only in the very last phase of his life this happened, perhaps you, like me, can let it go and neither blame him, nor yourself for having failed in any way.
Not easy, I know, but does it really matter now?
Illness, and the prospect of dying soon can do very odd things to the personality - try to see this as part of his illness.
Even if you have discovered that he had regularly deceived you for years during your marriage - try and let it go. You are only hurting yourself by lworrying about this. You may need professional help to move on from this, though.
As already mentioned: what proof do you have, bank statements, computer logs or gossip? Gossip is not proof, but someone's attempt to hurt you.