Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...

Each situation is different, but that doesn't mean you two can't heal. Both people need to be 100% invested in fixing the marriage. Have you two done any counseling, individual and marital? What do you mean when you say it is a dark cloud that you don't discuss.... you should be talking about this a lot, and as much as you need to, to help you feel better and comfortable and confident in his being now faithful.
Also helpful is to journal. Write and write and write about all your feelings, your hurt, your anger, everything. It helps to write it out and get it out of your head.
Try to do things that make you happy, outside of him. Talking with friends and family, doing things that you enjoy doing, hobbies, etc.
It's tough, for sure. I have been there. Like I said, it depends on the details... how quickly you can get past it.