I suggest you Google the effects of aspartame.... seriously.
Sugar free soft drinks are a lot more dangerous to your health than standard sugar laden ones.
If someone searches online for the effects of aspartame without having previously decided that it's a global conspiracy, they should see a very strong similarity with vaccines. Plenty of people claiming massively harmful effects because they read it or heard it somewhere and study after study after study showing none.
In some ways, the aspartame conspiracy belief is even flimsier - the vaccine conspiracy belief stemmed from a paper in a medical journal (with fake results, but it was initially in a medical journal), whereas the aspartame conspiracy belief largely stems from a hoax chain letter thrown onto the internet by an anonymous author ("Nancy Markle") which contains false statements and irrelevant statements (e.g. digestion of aspartame produces methanol, which is toxic...yes it does, but in tiny quantities, far smaller quantities than many other foods such as natural fruit juices).
The UK food safety body even did a study with people who explicitly stated that they suffered adverse side effects directly caused by eating anything containing aspartame. An absolutely definite link, with adverse side effects occuring every time they ate aspartame. In a double-blind study, those people showed nothing of the kind. They had no idea whether the stuff they'd eaten contained aspartame or sugar - the effects they had reported were caused by their own belief that aspartame caused those effects.
Aspartame is one of the most highly studied food additives and there's no reason to think it's harmful in the quantities that anyone could reasonably consume in food. The acceptable daily intake in the EU is set at 40mg/Kg and that's deliberately a wildly conservative figure, set at ~1% of the dose that might be dangerous if consumed daily. 1 litre of Pepsi Max contains 350mg of aspartame, so the OP's stated consumption would be 250-300mg per day. So they would need to weigh less than 7.5Kg to exceed the ADI and less than 750g to exceed the maybe possibly dangerous amount.
Unless you have phenylketonuria (a very rare medical condition), there's no evidence that aspartame is harmful unless you ingest implausibly large quantities of it.
I also seriously suggest that people search for the effects of aspartame...and not just on conspiracy belief websites.
I could provide links to some of the hundreds of studies done (some with sample sizes in the hundreds of thousands), but of course anyone who believes that there's a global conspiracy would dismiss anything that disagrees with them as being part of the conspiracy.
Large quantities of sweetened fizzy drinks may well not be good for you, but the aspartame in them is not going to cause almost every medical condition known to humanity (as the conspiracists claim it does).
Water would be better, flavoured if you don't like it plain. I just pour a small amount of squash in, much less than the ratio recommended because I'm only after a faint taste of it to overcome the faint taste of the hard water we get around here.
