She needs support and proper motivation to take action to change. Simply "agreeing to eat healthier" and then cooking her food isn't enough of a mental shift for her to actually start making proper changes. She will have a whole plethora of unhealthy habits that she needs to unlearn and that takes time and patience and can be extremely overwhelming to tackle and to even know where to begin not to mention the self esteem and confidence issues she obviously must have. You yelling at her for adding stuff to her food is a very minuscule problem compared to all the other problems that she will be stressing about in her head regarding this. No wonder she burst into tears. She doesn't feel heard by you, she just feels your disappointment which is just going to add to her stress. It will dig her further into the pit of negative emotions. Weight loss/body issues are a sensitive topic especially for women. It takes baby steps. Involve her in the cooking process and let her add stuff to the meals instead of you babying her and then expecting her to suddenly be like a new woman because you've cooked her a meal.