Sorry, hon - I am in the same boat! I am 39 weeks. I think there just comes a time when baby is so big, it's just always uncomfortable. This is my second and my first was the same - but I was 2 weeks late with him!!
I often end up laying on the couch with a pillow under my belly as support. Also, in a straight chair, try sitting with your butt a few inches from the back and then lean so your back is against the chair. It takes some of the pressure off your belly (but puts some on your butt!). You can't stay in any position forever but moving around helps!