If you can stay away from the artificial sweeteners with the diet soda (and anywhere else), I highly recommend it. I drank diet soda from when I was a teenager and in the past couple years it most likely caused a health problem I could have avoided. (I now also avoid artificial sweeteners in everything....the only exception is when I rarely go out to eat, I still have a diet soda.)
I tried to switch to sparkling water first, but they are really sharp! (That is, lots more carbonation than soda.) They are also pretty tart/sour, and I say that after having been off the diet sodas for long enough that my taste adjusted. Lately, I have found Spindrift "soda" (they also have sparkilng water). The soda has some sugar but not much (32 cal in the 12-oz can I'm drinking right now) and the carbonation is more similar to the levels in soda. It's pretty expensive, though.
For the protein, if you like cheese, then low-fat cottage cheese and string cheese is a way to get a lot more protein in your diet. Also, packets of tuna and/or salmon. Again, if they are foods you like. I also make a LOT of egg whites.