Why do i keep doing this?

I have been with my husband for 12 years and we have a beautiful family together. For the last year or so I have been browing the singles section on the internet. I never reply to an ad or even think about meeting anyone either. Sonetimes i just enjoy reading and looking at the profiles. I stop for a couple months, then I start doing it again. I know what I am doing is wrong and it is cheating. How can I stop this habit and y do I keep doing it? I have never not once cheated in 12 years but my husband cheated way before we married. I forgave him and he has been true to me ever since.