Oh, Sarah. You are so not alone in this. This whole mid-life transition is no picnic. I'm 51 and I've been able to hold my weight in check more or less, but there are definitely times when it spirals upward (that dreaded five pounds!) for no apparent reason. *sigh*
I identify with so many of the posts here. My experience mirrors Shevia's in that my weight seems to have a mind of its own, almost unrelated to how healthy I'm eating or how much I'm exercising. I follow her pattern of having unexplained weight loss windows, where I suddenly lose a few pounds. Then there are also those days when I wake up and feel bloated and pudgy all over for no apparent reason. It's difficult to predict. Carbs and sugar are definite dangers, though. And I do exercise regularly: yoga, weights, running, elliptical. And I do believe it helps.
Like Suz, I weigh almost the same (or even less) than I did ten years ago, but my body carries it much more in the waist/abdomen area. This is one reason I'm so belt-phobic...I feel like belts emphasize the body area that I least want to emphasize. It's so discouraging. I'm finding that I almost have to weigh less overall in order to have my clothes fit the way the did a few years ago. *double sigh*
You've gotten a lot of good advice about eating and exercise here, so I don't have anything to add except my story...and my sympathy. We are all dealing with this to some degree or another, and it's no fun at all.