I'm not really qualified to give advice, but- going from regular to diet soda will help. All those calories and carbs are gone (and that alone was pushing nearly 100g of carbs a day for me!) and you still get the caffeine nudge. Plus, if you hate the diet aftertaste, it'll dissuade you from soda altogether! You can still do fast food, you just have to stay smart about it- you can seriously get by okay with chicken nuggets and a side salad (if you're not going full keto yet, at least) or any of the entree salads that places have. (Suddenly Wendy's is my favorite fast food place, since their crappy fries don't matter anymore.)
Suggestions... apparently carrots aren't a great idea, as well as many large fruits- if you're doing low carb, anyways. But veggies in general seem to be better than fruits as the numbers go. Poultry and fish are good lean meats, but you'll want pork and beef periodically for the fat content. Again, I defer to the wiser among us for specifics.
Generally, if you don't have any kind of deadlines and you're looking long-term, just try to ease yourself into a new diet rather than jump in whole-hog. Your initial benefits won't be as big, but you won't feel like you're depriving yourself, and it'll make each stage of improving the diet easier to transition into.