My breast size is a 46DDD, and it causes me to have back pain.

I understand that you have a triple D size and you are having back pain. So you definitely need a breast reduction and a lift surgery. This can easily be done even if you are not married and I would encourage you to meet a board certified plastic surgeon near you and discuss the treatment options. The way we do breast reduction surgeries in unmarried females is that we reduce a reasonable number amount of size but at the same time we preserve their future lactation potential, so that they don’t have issues in their further life, pregnancies and lactation.

BRs which are medically necessary (i.e. you suffer back, shoulder, and neck "pain" with DOCUMENTED medical attention over a period of a YEAR) are typically covered by most medical insurance providers.  If not, they may try to deny you based upon a lot of things such as lack of symptoms, lack of previous medical management, and lack of enough tissue that should be removed. 
Read your medical insurance certificate of coverage. Determine what medical information is required for your insurance to issue a pre-authorization or predetermination of benefits for BR surgery. Then seek consultation with a board certified plastic surgeon who can evaluate your medical condition, discuss the pros and cons of this surgery as it applies to you, determine the amount of tissue to be removed, and help you in completing the paperwork required.  
Kenneth Hughes, MD, ABPS Board Certified Plastic Surgeon
Los Angeles, CA

Thank you for the question. Based on your description you may be an excellent candidate for breast reduction surgery.   However, much more information is needed to determine whether you should proceed with surgery, and if so when (timing).  In my practice, I individualize the “timing” of surgery based on a lot of information that is transmitted during the consultation process.
“Typical” patients who present for breast reduction surgery are women who have disproportionately large breasts, causing problems such as neck/back/shoulder discomfort, postural changes, bra strap grooving, skin irritation/rashes under the breasts, and/or difficulty with activities of daily living and/or exercise etc. There may be both physical as well as psychosocial “stress” caused by the disproportionately large breasts.
Reducing breast tissue mass and elevating the breasts on the chest wall tend to improve or alleviate many of the symptoms associated with the disproportionately large breasts.
Patients considering breast reduction surgery should also consider the potential downsides (risks/complications) associated with the procedure as well.. Poor scarring, for example may be associated with the procedure.  Additional surgery may be necessary in the short or longer term for a multitude of reasons.
When the time is right, I would suggest starting with the American Society of Plastic Surgery and/or the Aesthetic Society of Plastic Surgery to obtain a list of well experienced board-certified plastic surgeons.
Then, I would suggest you visit a few surgeons whose practices concentrate on aesthetic surgery. Ask to see lots of examples of their work and preferably speak/see patients who have had similar procedures done.
Then, it will be important that you communicate your goals clearly with your plastic surgeon and make sure that you have realistic expectations prior to proceeding with any type of surgery. In regards to breast size desired, also communicate carefully. In my practice the use of goal pictures are helpful in this regard; a discussion of cup size can be confusing and imprecise.   Therefore, I would not suggest that you communicate your goals and/or base your satisfaction with the outcome of surgery on achieving a specific cup size.  I also find that the use of pictures is more helpful than the words “natural” or "proportionate” etc., which can mean different things to different people.
Many of my patients choose to have enough breast tissue removed to help alleviate symptoms while retaining enough breast tissue to remain proportionate to the remainder of their torso.
I hope this helps.