Welcome to the forties! I think I can speak for most of us that we have all experienced some form of this. Worst of all it kind of just sneaks up and suddenly you realize you're not the same as your younger self...as depressing as it sometimes can be!
Everyone obviously reacts differently to a lifestyle change, but you will find one that works for YOU!
I personally have had the best success with eating every two to three hours and making sure there is adequate protein in each meal. I spend a couple hours each Sunday preparing all of my meals (for work) for the following week, put them in plastic containers and freeze them. I have managed to keep this up for seven years now and it's just natural.
Good luck! We are all here for you!