My DH loves Diet Coke too. When he starts drinking them he can't stop unless he goes cold turkey. We go through rounds of this each year. When he does go cold turkey, he gets TERRIBLE headaches. Ymmv, but just be prepared for some withdrawal symptoms.
He has replaced the dc with several different drinks per day. He drinks a lot of flavored sparkling water and zevia. We have a soda stream that gets used a lot. He keeps extra cans of zevia in his car in case he finishes one because they aren't widely available and he drives a lot. He also has started loving coffee. He uses the Nespresso machine that I got for Xmas a few years ago more than I do. Sometimes lattes but mostly he'll make double espressos. He also goes on kicks where he drinks tea all day. He seems to love Tazo wild orange because of the strong flavor. He will drink it hot or cold with stevia sweetener.
Replacing his Diet Coke with other drinks has really seemed to work for him. I'm sure it's healthier too. Good luck!
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