Replace them with an equivalent number of espressos and you should be fine. Or don't - it's the sugar in soft drinks that's the bad thing, the sweeteners are much safer, and caffeine's lovely, if you don't mind having an addiction. You might feel 0.1% better or something if you give it up [citation needed], but first you've to feel like **** for a while.
Diet coke has caffeine but not as much as a monster. If you reduce your caffeine intake it would be better for you.
See what you can do. Dehydration can cause head aches worse. Drink plenty of water too.
Keep us updated on how you get on.