Ok so I have been eating healthier, I cut out junk food completely but their is one thing I have not been able to cut out. I drink a soda a day, I have to be awake all night (my son requires 24 hour care so i'm awake about 20 hrs a day) and it helps me stay awake. I stay right under 1200 calories and exercise and have been losing weight steadily. Does drinking that 1 soda make a huge difference and throw off my diet or is drinking a soda a day while everything else is healthy ok?
My biggest concern would be that at about 1200 calories, it's really difficult to get all the nutrients you need from food. Consuming a couple hundred calories in the form of liquid candy makes it virtually impossible to get the nutrition you need to stay healthy.
One soda per day won't stop you from losing weight or throw off your diet, as long as you're able to stop at one.
I am concerned you are trying to stay awake 20 hours a day, eating only 1200 calories and part of that is pop. How fast are you losing? Could you maybe add a little more good food to your diet? I'm not going to tell you to get some sleep because I am assuming there isn't anyone else to take care of your son but you have to try to take care of yourself if that's what you're going to be doing.
To answer the question, one soda a day isn't going to kill your weight loss.
It won't ruin your diet, but it's not good for you...and it adds so much more calories maybe try something else to stay awake? it's hard at first but the cravings go away
I lost my first 75 pounds while still drinking about 1 can of regular soda a day. 1200 seems kind of low to be eating, especially considering a can of coke has about 150 calories. But, like I said it's doable if you're creating a deficit. I gave up regular soda for diet, it's a lot easier on my diet to not drink my calories. I don't think it's necessarily okay/healthy to drink a can a day, but at the same time I do think you can still lose weight while drinking soda...
what about a cafeinated diet soda, this is what I have (diet mountain dew). I only have one a day, I know it's not good for you so at least I don't have to worry about the calories. Your situation seems extremely stressful, is there anyone else to help you out?
You know, with everything you're going through, if the soda keeps you sane then drink it! As long as you account for it calorie-wise, you'll be fine.
As others, I would worry about your nutrition a little. How active are you? You may be able to eat more and still lose (say 1200 calories of food PLUS your soda)
The soda won't hurt your diet too much, but its definitely not healthy. Perhaps you could try replacing it with 0 calorie flavored sparkling water. Or if you really need the caffeine, opt for tea or coffee without loading up with flavored creams or a lot of sugar.
Just make sure you're getting proper nutrition with the low amount of calories you are consuming. Good luck!
Soda and calorie counting
maybe try something else to stay awake? it's hard at first but the cravings go away 
