You require less calories now, that's what it boils down to. You're burning calories at a slower pace, you've got to cut them back. One great way to rev up your metabolism though is through muscle training. It's vital for everyone - but for the over 40 crown even more so.
Adding in the strength training won't be enough though - you will have to combine it with a decrease in your calories.
If you are very wise and creative with your calories - lots of veggies, veggies, veggies, lean proteins (chicken and turkey breast, egg whites, fish), no fat dairy and some fruits, you will see that you can eat a lot of food.
I urge you to not focus on what you will be giving up, but focus on what you will be gaining. It is wonderful to lose weight in your 40's. As all my friends are bemoaning they're aging bodies - I am for the first time in love with mine. I'm actually way healthier and more fit and active and happier than I was in my 30's. I have no fear of my 50's.

