My questions are as follows and my answers follow:

Will giving up diet coke improve weight loss? I drink about 8 a day.
Possibly. But not necessarily. There are 3 things in diet colas that can be stallers: caffeine (which causes an insulin release), citric acid and aspartame. try cutting down by one per day. Substitute a glass of water when you feel you want #5
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Will antidepressants impede weight loss? I'm on 20 mg of Lexapro daily.
Maybe but not necessarily. Don't stop a medication just because you might lose more slowly. Dr. Atkins said that most prescription medications slowed the losing, but so do Tylenol & aspirin etc.
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If my ketosis sticks don't turn purple but only the slightest shade of pink, should I be worried?
No. Many people lose well and never show more than pale pink. ANY colour is a success.
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I have not cheated ONCE in a month. It's possible my carb count was off here or there but I'm being very faithful and of course, I'm learning more every day.
Good for you. that IS the way to succeed. Be patient during the days, weeks, months that your body is not yet ready to lose more. Tomorrow could be the day.