I am so sorry this is happening to you. This is the crappy part about making the choice that you did, and rarely do we see far enough down the line to consider the devastating result of the things we choose.
I have been through a divorce once, and it was a terrible time in my life. The main thing I found helpful was allowing myself to feel what I felt. I know that sounds silly, but seriously, if you need to cry, then do it. If you need to scream at the top of your lungs, do it! Basically, whatever you need to do to get through this time, allow it, as long as you are not hurting yourself, or someone else. This was very therapeutic for me during my divorce.
One thing I do want to point out is that you say that you made a huge mistake. Is talking to your ex hubby an option? Maybe let him know how you feel. It may be that he feels the same way, but is "hiding behind" this other woman to avoid his true feelings. Just a thought. An apology would probably be the first thing to offer and then just tell him your heart.
This is certainly going to be a difficult time for you. Get with your girlfriends and hang out. Get a new haircut, or a pedicure. Sometimes pampering ourselves is a really good thing to do to get through a rough spot. Whatever you decide, good luck to you. I know what you're feeling.
K.