Forgive if you still love him.
It is almost impossible to forget that he cheated on you.
It's not easy to get over the matter. Things'll be worse if it's ONS. Both of you have to wait for 6 months to confirm he's HIV-free.
If it's a mistress outside, maybe 3 of you sit down & talk it out. Who your hubby chooses & respect his decision.
It's his loss not to choose you.