Morgaine, don't do this to yourself. Don't allow your self hatred to overwhelm you. You are a good and valuable person, and your worth has nothing to do with how much you weigh. Nor with how many mistakes you make on your diet. You have to love yourself in order to be able to do for you, what you want and need to do.
Celebrate your successes! You took OFF 3.5 pounds! Hey that is great! So you ate some carbs and salt and now you have some water weight masking your fat loss. Does that mean that all that work was for nothing? I don't think so.
It isn't just your weight on Monday morning that matters. It isn't just one day of eating bad that matters EITHER. We can't be perfect people. We can't always resist EVERY temptation. And the good thing is that we don't have to be. We just need to be more consistent, more conscious of our choices for the majority of the time. You cannot gain 3.5 pounds of fat in one day.
That is a scientific fact. One pound of weight is 3000 or so calories. In order to gain 3.5 pounds of fat in one day you would have had to have eaten 10,500 extra calories yesterday over what you burned off.
How likely is that? It isn't. Instead what has happened is that the carbs, help you to retain water, as does the salt. That same water will go away in a day or two and your actual fat loss will show. Don't beat yourself up over a little extra water in your system, that water ISN'T a part of YOU! It is just pee you haven't peed out yet! Are you going to hate yourself or cry over a little pee?
Do you feel better yet?
One thing you might do, is something I read in the Carbohydrate Addict's book. What she did is to advise "weekly weight averaging". You take your weight every day, and then you average it over the week. By doing this you see your true average loss per week.
For instance you weigh 5 days and day one you are low, day two up a little day three up three pounds, day four down three and a half. etc. Add them all together and divide by the number of days you weighed. Then count your TRUE weight as the average for the week. And chart THAT number. You'll see a much more consistent downward trend in spite of any sudden spikes due to water retention.