I am glad you are able to talk about this subject online. It must be difficult to go through at your 'young' age and with a relatively new marriage. I am 59 and had been in a loving relationship with Ellen for nearly 16 years when I noticed my drive was not the same. I have not had IVIG or ITP or diabetes, and have no idea why my testosterone levels dropped. I have never heard of IVIG causing testosterone to drop.
I can imagine how autoimmune problems could damage testes, as they can damage almost every other organ, but I did read that testes are "immune privileged" which means they are less vulnerable to immune responses. Have you checked your thyroid recently? That can cause low libido.
Testosterone is just one factor, but it is an important one. I don't understand why you would need injections when topical treatments (Axiron, Androgel, etc.) work well to normalize testosterone levels. I have noticed my nails grew out stronger (it takes six months, though, for the nail tips to grow out new and strong) and my skin is a little less fragile.
Loss of libido is a common symptom of depression. If you have any signs of depression be sure to seek treatment. Some medications can help, but some, like the SSRIs, often cause a drop in libido.
If you are overweight, diet. Obesity is a common factor in erectile dysfunction. Your body will function better at a healthier weight, and I have always found that dieting itself helps to enhance libido.
Exercise. Get fresh air. Hike or bicycle or go to the gym. Get your heart pumping and your blood flowing. It helps the body as well as the mind.
Finally, focus your energy on 'the other 90%'.
I hope that others will share their experience and knowledge, and add to what I have said.
Above all, I hope you will take an ALL-OF-THE-ABOVE approach. Just one change might not be enough, but together they might bring about the change you are looking for.