The most common cause of daytime drowsiness (especially in Americans) is dehydration. You should try to keep a water bottle with you throughout the day and continue to drink a little bit at a time thoughout the entire day. Soft drinks are a large contributor to dehydration in Americans (since they tend to drink them alot and treat it as a water replacement.) So it's a good thing that you have cut down your intake of soft drink.
At first this will cause excess urination. This is not because you are drinking too much water, but rather because you are used to drinking too little water. You should adjust to the additional water intake over time and stop producing so much urine, as well as feeling better about yourself in general.
If your urine is generally a strong yellow colour it is a sign that you are not drinking enough water. You will find when you up your water intake that your urine has a more watery colour, do not be concerned with this.
Just on the issue of soft drink, I advise you against replacing regular soft drink with diet softdrink, as it has been documented to cause mood swings, headaches, etc. Personally I believe that regular soft drinks are better for you, despite the excess of sugar.
Sorry if any of this has been said already, I didn't have time to read past the first page as I'm trying to write an essay.