I am very sorry for your loss and I believe by acknowledging that such a traumatic event has had an impact on your eating habits then you have already taken a big step towards addressing this problem.
Ultimately, I believe that the psychologists appointment can be grouped into 3 main categories:
1) Your general mental health. For example do you have a mental health problem which might need addressing prior to surgery. This could come in many forms, such as pre-existing conditions such as depression, bipolar, anxiety disorder...
2) Your mental health surrounding food and/or body image. For example do you have or have had problems such as anorexia, bulimia, binge eating or body dysmorphic disorder which may effect both short term and long term recovery.
3) Realistic expectations. Do you fully comprehend what you are about to do. Do you understand REALISTICALLY what can be achieved from surgery. For example do you understand that you can actually gain wait post-op; do you know about the likely development of unsightly loose skin; do you understand that despite your body changing your mental state and problems you have now will not be cured; do you know that WLS is not a miracle cure; do you have an understanding of what your life will be like post-op - and how both your relationship with food and your personal relationships will change.
As has been already said on here honesty is the best policy. If your psychologist decides that you may need help with dealing with the loss of your son and its impact on your eating habits this may not necessarily stop you from having the surgery. As I understand it, both criteria 1 and 2 can be dealt with at the session and future sessions; however, as far as I am aware it is criteria 3 which could possibly prevent you from getting a recommendation from the psychologist. And don't get me wrong your surgeon can override their conclusions if they feel it is necessary. However, the psychologists evaluation is not only a tool for your surgeon to base his decision on, it is a tool for you to try and advance your own understanding of yourself and what you are about to go through; and the tragic loss of your son will undoubtedly impact on this.
Please don't stress or get worried about this appointment. Be open (as open as you feel comfortable with!), and do not forget that they are ultimately there to help you and have your best interests at heart.
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