Laura,
Getting to 42 with Cystic Fibrosis is a huge achievement. Add in diabetes on top of that, and I’m in awe of your strength.
When people with diabetes are losing weight because their blood sugars are sky high getting back into control will stop that weight loss. In that case the weight loss is because they are starving to death from not having any insulin to push glucose into cells.
But that’s the only time weight loss stops when you improve control. Most people with diabetes gain weight when they inject insulin and the more carbs they eat with insulin the more they gain. So eating more carbs would probably be helpful, except that with the other problems you have with CF you might not be metabolizing carbs into fat even with insulin. Also, the more carbs you eat, the and the more insulin you inject the more danger of hypos if you get it wrong, which if your digestive system is not working properly is a huge problem. So more insulin/carbs is NOT a solution for you.
Can you supplement with “good” fats like coconut oil. Quite a few people who have fallen for the idea that it is a weight loss aid report gaining weight eating extra coconut oil even when they are eating low carb. You can mix it with melted baking chocolate and sweetener and harden it up in the fridge and end up with something a lot like fudge.
Dr. Bernstein was asked in a teleconference last year about how to gain weight if a person is a Type 1 maintaining strict control by limiting carbs and he said to increase fat, so that would be my suggestion. The coconut oil is one of the more palatable ones, though I think the other health claims made for it are utter crap.