A month ago I found some evidence that my common law husband was having an affair and of course he denied everything.
Four years ago I met a man that I instantly had feelings for, my husband's cousin. We both hit it off really well but kept our hands to ourselves with respect for my relationship with my husband and family relations. Anyways, two or three weeks ago I caught my husband with his girlfriend, everything came out. So I moved out and have since contacted his cousin, and we have begun seeing each other, sexually but not publicly. I do believe we could have a very happy future together.
Now my ex seems to think he can ruin me if he finds us together. I do not feel guilty for being with his cousin at all, I am more happy now than I ever was in the 15 years we were together. Should I let this one go, or follow my dream of being happy and deal with the consequences of my ex?
Your problem is that you are an honorable person. This is why you stuck it out for 15 years in an increasingly unsatisfactory relationship, this is why you followed your perceived duty rather than your heart, and this is why you posted your question.
If what you wrote is right, then you have done nothing that your ex can use as ammunition against you. You have been faithful until AFTER you broke up with him. Once he and you ended your relationship, you were free. You do not need to meet your new man in secret. Broadcast your love publicly. Tell the world that you have yearned for each other for years, but kept apart because of your previous commitment. Now you are free of your ex, and here you are, with the cousin (this would read better if I knew their names, but you can do the translation).
If you have made the announcement, what can your ex do? He'll have to wear it, with whatever good or ill grace he can.