Overweight, 38 ddd breasts. Will going on a diet make breasts shrink without getting flabby?

Probably not. Breasts contain a variable amount of fat in addition to breast tissue. Significant weight loss will cause the fat and some breast tissue to shrink, however the skin usually cannot shrink in proportion, resulting in droopy breasts (aka ptosis). However, improving your health and maintaining a healthy lifestyle is the goal. The breasts can be addressed when you are ready.