You mention things that you can't have because of the diabetes; it sounds like maybe your doctor is treating you as a REGULAR diabetic and not a person who has CF related diabetes. I think that may be the large source of your problems with weight gain. You didn't say so, but is your diet low in carbs because of the diabetes? If so, you need to read up on CFRD. With CFRD, CF patients (you do still have CF affecting your pancreas even if it's not affecting your new lungs) are supposed to continue with a high protein high carb diet so that they can have a healthy weight. There are no restrictions on the diet with people with CFRD, they just have to adjust their insulin, a lot like adjusting the number of enzymes they take with higher fat foods.
Talk to your endocrinologist about managing the CFRD with a sliding scale of insulin. You need carbs! Don't avoid them.
I recommend you read this:
<a target=_blank class=ftalternatingbarlinklarge href="http://www.cfww.org/pub/newsletter/nl01a/nl01af.htm
">http://www.cfww.org/pub/newsletter/nl01a/nl01af.htm
</a>
p.s. I apologize if this is all information you know. I'm just trying to help.