As a mom of 3 kids, I have spent a LOT of time thinking about getting pregnant and trying to get pregnant. I turn into a crazy person sometimes when I'm thinking about wanting another baby. Much as you described, I can become consumed with thinking about wanting another baby, which really does feel crazy, considering how busy I am with the wonderful 3 that I have. Here is what I wrote to someone else going through what it sounds like you and I are going through:
I believe that it is in part biology (continuing of the species), hormones (which is also part of biology), and a craving for the magic thing that happens (at least in my family) when you introduce this miraculous new baby into your family and to all of the grandparents, etc. I add to it the amazing interaction between my children. They adore each other, and play with each other so beautifully!!! I really don't mean to suggest that it is just instinct that prompts us to want to have another child. It is very hard work, which you certainly know, and we would never want to do it just for the immediate gratification of having those 'newborn' feelings for a little while. It is an amazing thing, having a child, and I want to savor every moment of this intense, exhausting, wonderful, crazy time.
I don't know how to make it go away, but the advice you got about occupying your mind with other constructive interests is really good. I also found that talking to people (both on this site and elsewhere) helped me find an outlet for all that baby energy I was feeling. It also helped a lot that my husband and I decided that if we were going to have #4, we wouldn't start trying until the end of the year. That gave me a structure to work within, and I was able to give myself 'permission' to relax about it for a few more months.
I also want to caution you that at least for me, the second and third pregnancy are very different. Although I have been extremely fortunate not to ever have complications, the level of exhaustion I had chasing after my baby while pregnant was a surprise to me. I was fortunate to be able to have my little ones go to preschool in the mornings as I neared the end of my pregnancy.
Anyway, if you ever need to unload or just vent about what I call 'baby on the brain', feel free to send me a message. Chances are I'm struggling with it too. :O)