Keresley, I am sorry to hear you are going through so much pain. I had SPD, not nearly as bad as you, but I so understand what you are going through. I was postively depressed, I hated the pain, I hated my pregnancy, I hated dh for getting me into this situation, I hated other pregnant women for them being able to have active wonderful pregnancys and, sometimes, I even hated my unborn daughter for causing me this pain. I felt it was all so bloody unfair.
So,now the good news. Once the baby is out the pain will get a lot better ! It's only a few more weeks and THERE ARE THINGS YOU CAN DO TO EASE THE PAIN.
Firstly, run yourself a hot deep deep bath. I spent hours everyday in the bath that was the only place I was comfortable as I was in pain sitting and lying down as well. The water took the weight off my pelvis and the warmth of the water took the edge of the pain. Just be really careful when you get and out of the bath, sit on an edge and slide both legs in together.
Secondly, go to a chiropractor that specialises in pregnancy and childbirth related problems. She can find the cuase of your pain e.g. does it come from your pubis joint or your sacriol... joint and she will be able to manipulate your pelvis into a more comfortalbe postion. They cost a lot unless you are privately insured but my god, even one session, makes such a difference. In Germany e.g. all pregnant woman who have SPD pain are sent directly to a chiropractor by the midwife.
Thirdly, and I know this is controversial, discuss the option of an elective section with your midwife and/or consultant. With SPD you will not be able to have an active labour as you can't move around. You must not lie flat on your back and they must not, under any circumstances, put your feet into stirrups as this can cause your already week pubis joint to separate and then, sorry to sound black, you are in a wheelchair. The contractions alone will make your pelvis even more instable as it already is. The labourpain or an epidural will mask your SPD pain and you might move ina way that will damage your pelvis further, so all of this will limit your birth options significantly. So, really the only options are a waterbirth or a section. The fact that they can't guarantee you a waterbirth (no pool available, your baby goes into distress etc). I would advise that you discuss the option of a section.
I had a section with my dd and I am so grateful for that.
There is a good website to read up on: www.pelvicpartnership.org.uk. There's some useful information on it.
Good luck and IT WILL GET BETTER !