My mother was a SAHM for a long time, then went back to work after all us kids were in school full-time. She went back to teaching, then had to take more classes to get re-certified too. I was about 15 when this was all going on.
Not trying to sounds heartless, but when your 15 year old gets a drivers license, then that child can help run errands to the store.
One thing I've done is, if I'm going to be browning meat to make spaghetti, it's just as easy to brown 2 pounds in the same pan as 1 pound, then have 2 pots on the stove - 1 for chili, one for spaghetti sauce. Those can freeze easily.
There's a casserole that Paula Deen makes with the meat from a rotisserie chicken, 2 cans of cream of something soup, shredded cheddar (or some other kind of cheese), 3/4 cup of liquid (water or chicken stock or chicken broth), pour into a casserole dish, top with stuffing and melted butter (she actually tops with more cheese, but I like the crunch from the stuffing), and cook at 350 for about 35 minutes or until completely hot throughout (since the meat is already cooked before you start, you don't have to worry about undercooking it).
When you go to the store, they already sell shredded carrots, diced onions, etc., so take advantage of that too. Those rotisserie chickens are pretty good too.
There are things we get at Sam's - they have a thing called "click-n-pull" where you can place an order online, then go to the Sam's club you designate, and they'll have it all ready for you - just grab the cart, and go pay.
There are places where "kids eat free" on certain nights, or like Chik-fil-a, where they have "buy one kids meal get one free" nights - make sure you go on those nights to those places that you like that have such deals. First night I did that with my 5 year old at Chik-fil-a, We both ate for less than $4.00!! They also had a night where if you came in on a specific night, you keep the receipt from that visit, then come in anytime the next month, and get the same thing FREE. If our family goes, it's usually a $15-18 visit, so that was cool!
Some restaurants have a thing where you can put in your email to get "special deals". If you do that, I would set up a Yahoo email account JUST for places where you put in your email address. That way they won't all come in to your primary email account and you can go look at any of the deals when you want to.
Just a few thoughts - for what it's worth....and good luck!