Why do I want to switch partner every eight month? Why does infatuation not last?

After being in two relationships of over 8 months, I find that my sexual desire for that person drops significantly after the first 7 to 12 months. It gets to the point where I have to fantasize about other women in order to achieve climax.

Sometimes I think the issue is kind of a Madonna-whore complex. At first the sex is wild but gets more tame. Sometimes I think it's a fear of commitment.

Either way, this issue is coming to a head with my current relationship. I have been fantasizing about other women for over a year. I don't think I can do that any longer. To me, this means some serious choices.

I don't know what to do.