Getting upset doesn't founder a horse, but the history of super easy keeper and founder points to Insulin Resistance which is known to predispose horses to founder. You need to have your vet draw blood and send off a resting serum insulin test on her--this means no feed for 12 hours prior to blood draw.
For her diet, you are setting her up for gastric ulcers and poor dental wear with what you are feeding. You would be better off to keep her dry lotted but feed her a grass hay that is low in non-structural carbs (you can have it tested or just soak your hay for 30 minutes prior to feeding) and then give a ration balancer to ensure that all of her nutrient requirements are met because soaking the hay can leach some of the nutrient content. Horses NEED plenty of forage to ensure good digestive health and they don't NEED feed. So go for hay rather than a complete feed---it's better for her both physically and mentally. Whatever you are feeding needs to be low in sugars and low in carbs---no grains, no molasses, etc.
And daily exercise is really important.
Here is a link to a really good article: www.thehorse.com/pdf/aaep/obese.pdf