I can see how it happened. You are just in a horrible situation. I would seek a therapist to help you sort through your feelings. The truth of the matter is if you want your marriage to work you need to let go of the other woman. Completely and work soley on getting your marriage back together. If you are not going to work on your relationship with your wife you owe it to her, to let her go so she can have a life. If there are children involved then you should let your affair go and work on your relationship with your wife because it doesn't sound like there were any other issues that would make me believe that the marriage should end in a divorce.
The biggest problem I have with my wife is sex. When we are appart she works out and maintains herslef, so when I come home there is a lot of attraction and "spark" there. But after about 2 months she gets comfortable, gets lazy, stops working out, stops eating right and usually gains weight. It has nothing to with the weight alone but being married to someone that is so comfortable is playing hell on my loyalties and interst.
Another big issue is children, we both want them but she can't have any due to a horrible ectopic pregnancy during my first deployment to iraq.
No I do not think the other is due to convenience as it is most definitely not convenient, especially in the environment im in as well as the military holding adultery as a punishable crime unlike civilian law.
I would say it is convience for you. You get what you want when you want it even though your wife in theory is loyal to you. If you want a divorce get it, it will save you the headaches and free your wife to find someone that will not cheat on her and possibly bring home a STD.
If the problem is children then adopt. So many kids need a good home.