I think these things happen because you have been with DH for a while and have kids so you feel kind of trapped even though you chose to be with DH and chose to have children (I'm assuming you did).
Because you feel trapped you want to make sure that what you have got is the best and you aren't missing out on something better you could have had.
I felt this way about another man and I don't know him all that well. From what I do know, logically I can see that if I was with him there would be problems, because there are lots of things 'wrong' with him, but I felt blinded by love/lust. I visualise him being so wonderful and perfect if we were together and I now know that this is JUST a fantasy.
A thought that made me feel loads better was "Do you know of anyone with a relationship that you would rather have than the one you have got?" I don't. This must mean that this mythical perfect relationship we fantasise about probably doesn't exist. If you left your DH and got together with the OM, after the 'honeymoon period' you would start to notice his faults as well, and at best you would only get into a similar relationship with him to the one you have with DH. It isn't worth it is it? Unless you are really unhappy with your DH, and it doesn't sound like you are.
I've tried to accept that if you find your DH 'acceptable' and he shows you love and provides companionship then this is enough to be contented with. Another thought that helped me was "Would you rather have really intense passion and excitement with OM which lasts a short time but then have nothing when honeymoon period is over and having lost DH and DCs, and then you would have to find another man and do it all again for the same 'rush' and again and again etc, or would you rather have a lifetime of less intense love, and support and companionship and someone you feel you can rely on and stability and relax about it?"
Just as you are seeing your DH's faults more because you have been with him a while, he is probably seeing yours. But he is staying with you because he loves you. This is worth a lot. Don't look elsewhere, be reassured that it doesn't get any better!