For what it's worth I gave up my 2-3 bottles per day of diet Mt Dew first week of January and can't believe the difference in how I feel.
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After this I truly think the stuff is poison and screws up your system.
I did the same, nearly 20 years ago, going back to sweet tea. Another thing to convince you:
Let a soda go flat - the carbonation is misleading. Once it is flat, pour it off into a glass, taking notice of it's syrupy consistency. try to approximate that consistency in sweet tea or lemonade by adding sugar. Then try to drink the result.
Folks roll their eyes when I add sugar to my tea, but I know for certain that the sugar I use is far less then in that soda they're drinkin.