IOA I think that so many of us have been there! I was a SAHM for 7 years, and in a lot of ways they were the most difficult of my life so far. Part of it was that I just felt so incredibly unproductive. I would see my husband work and accomplish things, and I was terribly jealous of him. I missed intelligent conversation so much and just felt lost. Of course, none of us moms should really feel that way while we're at home. We're doing an important job, it just doesn't feel like it to us while it's going on.
But it doesn't last forever! Or at least it doesn't have to, if you don't want it to. Last year, when my youngest hit all-day school in first grade, I went back to grad school full-time. I decided to become a high school math teacher, and right now I'm doing my semester of student teaching. It is really difficult...I'm having a lot of trouble balancing my workload and spending time with my family...but it gives me another purpose and a great sense of accomplishment.
BTW - I'm young too, and I never had that period of career success before quitting my job. I hated my old job (actuary) and knew that I wouldn't want to go back to it after the kids were in school. Now that I have a new plan, I can say that I am hopeful and excited about the rest of my life.
