It sounds like there are a lot of issues you are dealing with in your marriage. An affair does not happen in a bubble, there are many contributing factors. You are human and you made a mistake, but realize an affair is a symptom of other issues. You mention that in the marriage your husband has deal with addictions and has been neglectful, these are not minor issues and do not make for a happy marriage. This is not something the two of you should go at alone.  I highly suggest getting counseling. Not only do you need to heal from the affair, but you need to address the other issues that exist in the marriage. You also need to learn effective boundaries, friendships that turn intimate is the most common form of an affair. You were lonely in your marriage and you turned outside your marriage to deal with it. Now it's time to turn into your marriage and work together at understand what happened and why and begin the process of working through the issues that exist beneath the surface.

Thanks for the advice. My husband thus far has been opposed to counseling, but today he said he would go. I feel hopeful- a little bit now.

My husband and I went to the LovePath 911 workshop last weekend and it was so good for us....  I can't say it was easy - far from it - but it helped us so much.  Not just in understanding why I had an affair, but in giving us great hope in moving past the affair. 

I, too, was horrified I had an affair.  After 20+ years of marriage I thought I was immune....  it happened and it was traumatizing to our marriage, but the workshop helped us see that our marriage can get back on the right track and be stronger than ever.... 

Good luck to you and your marriage!