Work on your psoas muscle

As a former LMT, I know that this is one of those almost impossible to reach muscles that - along with the Quadratus Lumborum muscle mentioned here too - truly affect the low back. See if you can find an LMT who has good experience with the psoas, find stretches that work on it, or if you can put up with the discomfort, get yourself a tool that can access the psoas like the Pso-Rite or the Hip Hook.