I think I have periods where I am very happy and others where I wonder "is this it?" I don't think it's abnormal to feel unhappy or unsatisfied occasionally in a marriage, it cannot be passion and romance all the time, it just depends on what you are willing to live with.
I was in an emotionally abusive and eventually physically violent relationship for seven years, followed by a horribly intense relationship with a pathological liar who was also wooing a number of other women at the same time (and sleeping with one, I later discovered). When I met dh, I couldn't believe how supportive and well, nice, he was to me. I finally realised what a relationship should be like. I kept waiting for the put downs, the belittling, they never happened.
Unfortunately for women who are used to abusive relationships (my dad was a cheater also) when a nice, genuine man comes along we can find them boring. No drama, no pain, nothing to fight for - this often equates to no passion. I have to keep reminding myself that a healthy relationship lacks constant drama for a very good reason - it is not normal.
I don't know if your background is anything like mine, but if you have any kind of abuse in your past, then the way you feel is very common.
Sorry if I'm assuming too much - I'm a listening volunteer for a charity now and I hear this a lot from women with abusive pasts, but I also hear the same from women who have changed over the years and no longer love their partners in the same way.
I hope you can figure all this out, happy to chat any time x