What you need is willpower. No way around it. Your options are:
Drink soda sweetened with sugar
Drink soda sweetened with artificial sweeteners
Drink unsweetened soda
I would just stay on the seltzer until you’re used to it. I can’t even drink sugary soda anymore, maybe as a treat once in a while, but never a whole can or bottle at once. You’ll get there too.
Summer. The best activities: dining on patios and drinking in beer gardens, camping and hiking, going for long walks, and lots more. Long days. Heat doesn’t bother me, and if it ever does I have A/C.