Many of us can relate to how you are feeling. Try to take heart in the knowledge that what you feel is entirely normal. And although it does take a long time to heal from the damage associated with affairs and the underlying problems in your marriage, there is a good chance that you can achieve this if you try.
Remember to control the damage as much as you can. Don't ask questions unless you are prepared for the answer. It's a bad idea to give your W the power to decide what you will know or not know about the OM. I recommend writing your questions down first, then thinking about how you might feel about the answers (best case/worst case) for a few days. If you still want to know after that, then ask away. If your W is hesitant to provide info, you may want to have her read this article at the Peggy Vaughn's website (the author of the Monogamy Myth):
I recommended the Marriage Builders site because, unlike many of the books you might read, including DB and DR, the Harley books recommend a specific approach to affair recovery. There are similarities between the approach taken by MB and that recommended by DB, but in the initial stages of recovery, I found MB more helpful. For me, it was impossible to conceive of working on the marriage until the affairs were stopped.
I highly recommend working on yourself in the meantime (take Michele's good advice), but you should also take specific steps to assure yourself that the affair is not continuing during the healing process, IF you want to save your marriage. The MB program offers specific steps designed to stop the betrayal - they may not work all the time, but at least it will give you something to do. The DB program, IMO, offers some of the best advice on working on yourself and rekindling your spouses interest in you. Whatever you decide to do, take control of your life. Do not let your W's despicable behavior and emotional turmoil associated with it control you. Get a life and start living it - go out with friends, exercise, read for fun, start a new hobby. Don't let your W walk all over you emotionally (and don't do it to her either).
Above all, do not make any hasty decisions about your marriage until you've had time to sort through some of the most troubling emotions. Give yourself at least six months before you decide whether or not to divorce her. That's really not a long time at all, especially if you really apply yourself to getting out and having fun. You can also work on being the best possible H to your W you can be (whatever that means for your marriage). This will be beneficial even if you eventually decide divorce, because you may learn important lessons you can apply to future relationships. Keep in mind that the divorce rate is higher for 2nd and 3rd marriages, probably because people keep making the same mistakes over again. So, learn to stop making those mistakes in your current relationship and you will reap the benefit no matter what the eventual outcome with your W.