Livi- firstly, you do not 'have' to do this for your loved one. I am sure they wouldn't want their memory to be something you beat yourself up with. You are doing this for you, knowing you have their love and support with you REGARDLESS of how well you do.
Secondly: I can relate in a slightly different way. My husband was diagnosed with cancer last October, 2 months after I got to target. The 'old me' would have used the stress, horror and general despair as a reason to overeat. I was determined this time that I would not allow such a horrific thing to 'give me permission' to reach for the chocolate. Watching my husband go through radiotherapy every day, I thought it the least I could do to stay strong and foccussed and not just wallow in twix bars and bombay mix (I had a few tiny lapses, but generally I fought hard)- it isn't helpful or constructive and despite what we tell ourselves at the time, it doesn't make us feel better.
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