How do I fix my flat sourdough loaf? I have made sourdough bread several times using roughly the same technique that I use when I make artisan bread. I let it ferment several times folding between them.

I have found that when the dough has risen enough it is so acidic that even a gentle fold causes the top surface to pull apart. When the dough is risen the last time as a boulle it relaxes so much that it practically flows. The resultant bread is too flat and sadly dense.

I have experimented with adding more flour to provide more structure but this only make the loaf tougher- not higher or more open. I have tried with AP flour and various wheat flours and combinations thereof. Do I need more gluten? Would adding vital gluten be enough?

How should I change my process to create a sourdough loaf that is more open and can rise higher?
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.