Covering the faults of others

I did nikah with a woman few days before. The very next day from someone outside I find out that my wife had physical relationship out of marriage with a man for 3 years. I went home and asked her straight and she confessed that she had physical relationship with him so much so that they had done kissing all over, seen each other naked and touched each other's private parts (masturbated each other) but she made promise on Allah that they did not have intercourse. I can't believe that they did not do intercourse, even if they did not do intercourse how a husband is supposed to forgive all this? If I leave her the 25 years of relationship of two families will be destroyed. But I can't forgive her. I try to forgive her but whenever I see her I imagine the two of them having doing zina and that is unbearable for me. Help me what I'm supposed to do. Even if I forgive her how can I cope with my imagination?