K8ee I'd suggest your midwife has no idea what she is talking about! It sounds like you DO have PGP (same thing as SPD btw). Your symptoms are exactly the same as mine and I've had it since the 17week mark.
She really ought to have referred you to a physio earlier 
I'm 37 weeks now and unfortunately because the baby is dropping into my pelvis the pain has generally become a LOT worse. There isn't a lot my physio can do about it at this stage so we just have to wait until the baby comes but I'm on the books until 6 weeks after birth so if there are any post natal problems they can look into sorting that out.
Heat definitely helps and the main thing that is allowing me to sleep as night is getting DH to massage my lower back, hips and down the back of my thighs (my physio used to do this on a weekly basis). Is yours able to do this for you? If not can you book yourself in for a massage? It really makes the difference between me sleeping solidly for 2 hours at a time or having to wake up and walk around every 45min to ease the pain/stiffness. Sleeping with a pillow between your knees helps keep your pelvis/hips in line so persist with it even if it feels uncomfortable.
They say PGP can make birth easier because in theory your joints are looser so it is easier for the baby to come out. But I don't think any serious medical studies have been done on this and the information is purely anecdotal.
Even although you are able to open your knees quite wide you should still make your midwife aware of your pain during labour because you don't want to overextend the joints!
Water births (or just being in a birthing pool) is said to help PGP sufferers a lot because it helps take the weight off the pelvis/hips so I'd ask for one if I were you - hope you don't have too much longer to go!
There is lots of helpful information on it here