Hello Di,
I'm also not Cathe, but I can surely relate. I'm 49, also 5'7", and weigh 147. I have lost a lot of weight, but I still have what I refer to as "my bosu" (do you know what those are, those exercise platforms that are like half-dome stability balls?): it's this little soft pouch just under my navel whose shrinkage defies any and all abdominal exercises. My workout regimen is to do aerobic exercise one day, strength training the next, and alternate in that fashion each and every day, with only the occasional day off. Fully 99% of my workouts are done to Cathe DVDs.
HOWEVER: My belly bulge has recently shrunk by 1.5", thanks to increasing my protein intake. My husband and I are "pescaterians" (or "vegaquarians") in that we eat a mostly plant-based diet with a little fish and seafood. But when I did the math I realized that I was getting less that 10% of my calories from protein of any source --- way too little. So I increased it to between 15% and 20% (which amounts to between 56-75 grams of protein daily); I stick to about 1500-1600 calories a day, which allows me a bit of a binge on Friday.
I can honestly tell you that, making no other change in my workouts, this has finally caused me to lose inches in that critical area. And it's only been two months since the change, so it didn't take forever to see the difference. As an additional benefit, I don't have nearly the weakness and muscle soreness I'd experience when doing strength training. Mind you, I still find that when my Friday binge includes a lot of carbs, they surely result in temporary belly-bloat, so I don't do that nearly as often as I used to.
Your mileage may vary, but I hope this helps some. I'm still losing weight and inches, thanks to the dietary change, so maybe it will work for you too.
Best of luck,
Rebecca