There are several things that occur. Both mentally and biologically.

Mentally you are denying yourself a variety of tasty foods you like and you will eventually give up and eat the way you did before putting on more weight than you had before your diet.

Biologically your body is starving for all the essential nutrients thus it will either shut down (coma or death, that is extreme like anorexia) or start storing everything you eat thus gaining weight. Many people who put themselves on a very restricted diet will find that initially they will lose a lot of weight in the first few weeks then all of the sudden start gaining.

I know you don't want to be fat. My parents are fat and I refuse to ever get to their state. It takes a lot of will power, but don't ever beat yourself up if you slip at a meal. We all indulge every once in a while and that is ok. A good diet comes from habit. I changed my eating habits in 1998 and yes it was rough at first, but over the years it has become easy.

What you are drinking is fine. I drink one cup of coffee in the morning, a tumbler of Tang, a couple cans of diet cola, and the rest water. A few times a year I'll have a couple glasses of wine, but isn't something I would want to drink daily.