I'm sorry you are suffering, it is horrible.
As pps have said, check out Pelvic Partnership. Their website is brilliant and they are very helpful giving advice by email too.
Keep your legs always a little bit apart but not much. As has already been said, pillow between legs in bed, small steps, stairs one at a time, and so on.
Definitely put it on your birth plan and be sure to get your birth partner to remind each midwife and doctor.
Look into osteopathy privately.
Push your GP for any help they can give you.
This has made me sad reading this. I won't tell you my whole story as it's not very encouraging for you but suffice to say a year post partum and still really struggling with it. Please do seek hell and take on board all the advice you have had here. Whatever you do, don't assume it will just disappear post partum, and don't suffer in silence or push yourself, you must look after yourself!
Finally, I need to correct a pp.
One thing the books do not tell you is that breastfeeding sustains higher levels of relaxin hormone post-delivery. The specialist women's health physio told me this on my second pregnancy (had SPD from c18 weeks with both). She also advised against fast weaning if you decide to stop breastfeeding, as it drops the relaxin hormone levels so quickly it can increase pelvic joint pain significantly.
This is completely wrong!!!
Unfortunately it is a common misconception amongst doctors and others.
SPD is a mechanical problem (ie pelvic misalignment). It is not due to hormones. Stopping bfing will not change it. Please don't make decisions about bf because of your SPD, it is entirely separate. SPD needs manual therapy (osteopath or some physios but not all).
Hope that helps and good luck for a healthy(er) and happy pregnancy, birth, and parenthood.