I'm with MLAN, you need to figure out what spot it hits before you can figure out how to replace it. Adding raspberries to your water will do nothing as far as a caffeine fix goes. I will also say to start where you are and play around with one diet soda a day until you find something that you like. This way you're mostly keeping everything else the same and then just trying alternatives until you find something you like.
It may be helpful to figure out why you like the diet drinks. That may be key in figuring out a substitution. Is it the fizz? try sparkling water. Is it the caffeine? Try tea (black, green, white, oolong have caffeine) or coffee. Is it the flavor or the chilled drink? Herbal teas and true teas (mentioned above) have a lot of different flavors and can be served hot or cold,
I gave up diet soda too & I feel much better for it. Cravings are far fewer now. I like to keep a pitcher of water with fruit in it. The varieties are endless from basic lemon or lime to mixes of different fruit. Sometimes I throw an herbal fruit teabag in the mix which makes it look more like a fruit drink. It is really refreshing & even my hubby has come to enjoy it.