I use Crystal Light with Splenda - but only occasionally as a drink. Really it's because I like making decaf black tea cold as my 'iced tea' (I just prefer it). It's taken me a LONG time to bump up my water to a level that it's appropriate. I can suck back 4-5 litres some days now without thinking (big girl, need water). When I first started it was tough to get what IP wanted me to get. I drink one pot of tea in the morning another in the afternoon and a big glass of water with lunch. That gives me 2.5 litres and I'm not home yet.
It will happen, I loose better when I'm drinking my water. I had a week of -2 and my coached mentioned the water so I tested it by really concentrating on it. I lost -5 the next week.
Keep trying, use the Crystal Light with Splenda though. It will happen!