would you please clarify what medication is she on and if she is receiving therapy?
Thank you for clarifying. Her behavior is driven by something that servers her (even if it is unhealthy or "wrong") Some speculations- 1/ she may be trying to avoid these diagnoses and not think about them or that they in a way control her life, hence telling you what you want to hear 2/ she does not want you to worry and wants to please you 3/ she is struggling with this regimen (maybe she feels odd or compares herself to others= self esteem issues) 3/ she could have felt in control of her own life at age 16 and thinking what she does is best for her (in a way trying to take control of her health and life the best she knows how to)
It may help to find out from her how she perceives your expectations and if she has issues with them where do they stem from. do these have to do with her frustration regarding her diagnoses, does she feel you're trying to correct this for her or to control her, does she feel stifled, etc. Talking to her and finding out her perspective and what fuels it may be step one for you. Then, you would want to re-educate her as to the reason why you're asking her to do this. Third, you can let her know that you cannot watch her 24/7 and would like to have some trust in her that she will do what is best for her health regardless of whether she agrees about this during this time of her life.
You may let her know that the only consequence is her health on the line and your level of trust that the two of you share. The conversation can focus on empowering her (what you know she's capable of , what is good/healthy for her and in strengthening your mother-daughter bond and trust level) Punishing her for this now would be counter productive and you know she can do and tell you what she wishes. Try to find out what is causing this resistance in her. Does it have to do with taste, routines, reminding her of her issues, etc. and take it from there.