Double bubble - is this a severe case, and how can it be fixed? (photos)

I am sorry to hear about/see the problems you are having after breast augmentation surgery. I think your concerns are  appropriate; your breast implants do seem to sit quite low on your chest wall, consistent with breast implant displacement ("bottoming out”).
It is probable that you are feeling the implants significantly along the lower poles of the breasts. Patients in your situation often complain of discomfort in the area of implant malposition. Also,  consistent with the breast implant positioning problem, the nipple areola complexes seem to be sitting relatively high, because the breast implants have settled too low.  
I think you will benefit from revisionary breast surgery which in my practice  would likely involve capsulorraphy ( internal suture repair).  This procedure serves to reconstruct the lower poles of the breasts and prevent migration of the breast implants too far inferiorly.  Associated issues with positioning of nipple/areola complexes  should improve with this operation.
Make sure that the plastic surgeon who does this procedure for you can demonstrate significant experience with revisionary breast surgery.I hope this, and the attached link, helps.

The line or double bubble that you're seeing is the insertion point of the pectoralis muscle. The thinness that you are experiencing down below is the fact that you have very little subcutaneous tissue between your skin and the implant. There are several corrective options. One would be replacing your implant above the muscle. Another would possibly be using Seri increase the fibrous tissue layer between your muscle and the Chestwall. I'm not sure that fat injections will correct this problem. It would be important to discuss this with your plastic surgeon and discuss options with him second opinions are sometimes advisable.