How do I fix my flat sourdough loaf?
I suspect your sourdough starter is just too acidic.  I have no similar problems with sourdough in the past, but my starter isn't particularly acidic.  

Have you tried discarding with a large refresh and then shortly (in bakers terms) trying a new loaf?  Trying to isolate just the yeast and not the sour part.  The loaf may not taste sourdoughy, but it should determine if that's the issue.