ibcrewin has the right idea, make sure you are eating enough calories to backup the amount of exercise your doing. It is good to want to burn up fat supplies with exercise, but that takes time, if you're not eating enough, your body will shut down and you will hit a wall in exercise and weight loss. It seems counterintuitive to eat more when you are trying to lose weight but that may be why you are getting burned out and having to slow down when going up the stairs. sounds like you are already eating the right stuff, just think about eating a little more of it.