Are you DBing or is your husband? Are you seeing a therapist to save the marriage or for other issues as well? I ask because I want to know if the counselor is solutions oriented toward saving your marriage. I am glad to hear your husband makes you feel happy when you are together, but you probably wonder if his changes are real and for the long term. He seems to have a pattern of disappointing you. Why do you sweep what he does, as you say, "under the rug?" Are you afraid of being alone?
Your feelings for the OM are certainly understandable, but keep him away.
I haven't heard you mention children...do you have any?