I think the best thing you can do is realize you're one person and you are doing all that you can. When your children grow up they won't care that your house was messy - they will care if you were there for them. As a single, full time working mom of two girls, I can tell you I'm always on the hunt for quick low cost healthy meals. I don't have a lot of good ideas for that except that I tend to shop a lot of the clearance area and base my meals around what I find there. Anything that freezes well I buy in bulk, especially on sale.
As for finding the time for everything... just do the best you can. So what if your house doesn't look like Martha Stewart's lol I found the best method for me was to spend time with the kids, in corporate them into some chores, and then get them to bed at a reasonable hour (which they will appreciate anyways!). Once they're down, I designate a small portion of time to cleaning, a small portion to a little work out, and decide mentally what I'm going to make for the next day to eat. Then the guess work is gone, even if the prep isn't.
Also, drink water like it's going out of style. There's few things less productive than a dehydrated single mom. Try and get in the habit of taking a multivitamin every day (I was completely against taking them until I got in the habit - now I swear by them!) I also boost mine with extra vitamin D and vitamin C since they're water soluble.