When you catch yourself saying I can't exercise, or I can't pass up a chocolate doughnut, change it up to "I Won't." By saying "I won't" do something instead of "I can't" forces you to hold yourself accountable! "I can't" gives you an excuse; "I won't" says I am responsible for my own actions.
When you say I can't, that implies that you do not have the ability. If I say I can't exercise tonight, I have given myself an excuse. If I say I won't exercise tonight, that is my decision. I won't thereby turns into, I won't tonight, but I'll fit it in elsewhere-a walk at lunch perhaps.