thenewme, you know you're being unfair to your husband and that's why you feel so bad, so I'm not going to lecture you. It will be you who gives you the hardest time over all this, no one else. Guilt is one of the most difficult emotions to live with. I sympathise.
I think you need to do some long, hard thinking about what you want. Is there something missing from your marriage that you think you can get from this OM? If so, is there any chance you can work on your marriage to get that from your DH? If it is the OM and no one else will do, I think you need to think seriously about whether you can truly live with yourself if you stay in the marriage or leave. If you stay, you may ruin your marriage anyway. Unless you make the effort to get over OM you will destroy all intimacy in your marriage and eventually you will hurt your DC as well as your DH. IF you leave, however, there is the very real possibility of finding out that the reality is different to the fantasy, and it's possible (even if you and OM are a match made in heaven) that the guilt you would feel from leaving your DH will destroy your relationship with the OM.
In your shoes, if there is nothing actually wrong with your marriage I would choose to stay. However, this problem will raise its head again unless you work on it. That means cutting all contact with the OM, which you already know. I think it would also benefit you to go to Relate, alone. They have a lot of experience about dealing with something like this, and the fact that you would be seeking help before actually committing infidelity means there is way more hope of fixing your marriage.
Sometimes it is possible to be with a thoroughly decent husband or partner but then later discover your soul mate. It happens. It's no one's fault. You can't be condemned for your feelings. However, bearing in mind that nothing has happened with the OM and that your relationship with him cannot be that intimate (emotionally) because you're restricted to spending snatched moments with him, I think you have to accept that you simply don't know him well enough to intelligently decide that he is your soulmate. In those circumstances I think you owe it to your DH and DC to work at fixing your marriage first. If things fail to work out, at least you will suffer less guilt because you will know you have given it your best shot.
It's also worth remembering that life is about much more than romantic love. Children, friends, hobbies, even work all contribute to make us the people we are and all can give us happiness. Life doesn't have to be unbearable because love is missing. Work hard on making a good life for yourself and the pain of missing the OM will slowly fade, especially if you work on your marriage as well.
Hope you feel better soon.