How much do a woman's breasts change between 17 and 23?

I am sorry to hear about the problem you are experiencing but online consultants will not be able to help you with predictions regarding breasts size changes with ongoing weight loss.  Having said that, based on your description, you may be a good candidate for breast reduction surgery at some point; best to achieve your long-term stable weight prior to proceeding. “Official” recommendations of course would necessitate in-person consultation.
Breast reduction surgery is one of the most patient pleasing operations we perform. By removing “excess” breast tissue, adipose tissue, and breast skin this operation reduces AND lifts the breasts to a higher position on the chest wall (in other words, the breasts are "automatically" lifted when a breast reduction is performed). By doing so, patients often find improvement in neck, back, and shoulder discomfort and find it easier to form their activities of daily living and exercise.
Timing of the operation will depend on the patient's life circumstances.  If/when the time is right, seek consultation with well-trained/experienced board-certified plastic surgeons. Ask to see lots of examples of their work and communicate your goals clearly. Best NOT to communicate your goals in terms of achieving a specific cup size; this type of communication can be a source of miscommunication. In my practice, I prefer the use of goal pictures during this communication phase; many patients wish to remove enough breast tissue to improve/alleviate symptoms while maintaining upper/lower body proportionality.
I hope this, and the attached link (dedicated to breast reduction surgery concerns), helps.

Dear pinkeose064,It sounds like you have had your fair share of changes already over a short period of time! When breasts enlarged with weight gain or pregnancy they don't always bounce back to the same size because of the excess skin and droop which adds cup sizes, but not necessarily a lot of heavy weight. Remember too that as your band width decreases on your bra with your weight loss, the volume of the cup size also decreases. The volume in a 38G and 36H would essentially be the same amount, so while the letters change, your volume may not. You may have already lost the fat in your breasts and now it is the residual skin that is giving you the extra volume.If considering surgery be sure your weight is stable and your health is good. Having surgery before you have children may negatively effect your ability to breast feed successfully, and again you likely may see some changes to the size and appearance of your breasts like you have already afterward. An exam and getting a sense of your preferred breast size would help determine if you would benefit more from a lift or reduction, which are very similar procedures that differ in the volume removed. This attached video may help.Good luck!