One of the simplest and cheapest things you can do to build core strength (gently) is walk. I think a dr. Visit is in order, sounds like you need to be checked out. Can I ask, why 'toast' is a snack? While it would be better than nothing, a snack should include a little protein. A glass of milk, yogurt, string cheese w/fruit or even a small handful of nuts.
Take care of yourself, and be smarter than some of us (older) people were in our teens. The body you have now has to sustain you for a long time ; )
I did have a doctors visit (forgot to mention that) and she confirmed that I have exercised-induced asthma as well. So, between using an inhaler when I exercise and staying on a gluten-free diet, I'm really improving quickly. And Grace is, too. She's making leaps and bounds of progress, and thankfully I'm now strong enough to carry and help her, and I'm getting stronger :)