I do not have diabetes, but I can empathize with you.
I come from a family of naturally thin people, but unfortunately did not get that gene. My natural body -not pregnant or breastfeeding- requires about 1,500 calories in order to maintain my healthy weight. Anything more than that, and I can put on pounds very quickly.
I am pregnant with baby #3 and have breastfed both of my daughters for a combined time of 27 months. While pregnant, I increase my caloric intake 300 calories like recommended, so I am eating 1,800 calories per day. While I breastfeed, I eat between 1,800 and 2,000 calories a day depending on my appetite. My pregnancies have been very healthy, and my milk supply has been abundant with both of my children; as long as they demanded, my body supplied.
Have you spoken to your doctor about your post-pregnancy/ breastfeeding diet? I wonder if they will increase your caloric intake to 2,000 calories. While you are pregnant it is recommended you increase your intake by 300 calories, and while breastfeeding 500 calories.
It is most important with a lower calorie diet, that the majority of -if not all of the calories you consume come from healthy food sources. Avoid empty calories like candy and soda. Not only will it positively effect your/ you baby's health, it will make you crave those sweets even less. Your body starts to crave an apple vs. a bag of gummi bears.
For your future concerns about developing GD in a second pregnancy, I would say you are absolutely on the right track already. Being aware is the first step. You have taken action by changing your diet, which is the second step. As long as you keep up your good work, and continue with a healthy diet post-pregnancy, I would think everything will be okay.
Keep in mind that even woman who are not over weight can develop GD. If you are following your regimine, and develop GD in your next pregnancy, do not blame yourself. Some woman are more suseptable to developing it during pregnancy.
Good luck and congratulations on your soon-to-be motherhood!