I would try some resetting of the femur head into the back of the hip socket. if your femur head is biased forward in the socket, then you will be restricted in hip flexion as the femur rotates up it will run up against the top of the socket.
I would look at both of these ways.
the first is to get on the floor and do Hip Capsule External Rotation. use a band to distract the hip to the side or rear. then as you lay towards the ground, try to drive the femur head out the back of your butt. do this for 2 min on each side. retest hip flexion – standing i assume?
the second is standing, and let a band distract a hip towards the back. then bend over the banded leg and do hamstring flossing while letting the band drag the femur head to the rear. retest hip flexion after doing this.
as for psoas, there could be many symptoms of a tight/weak psoas. one is if you are in a extended low back constantly. the psoas could be dragging your low back forward as you try to stand straight up. glute inhibition could be another. limited hip extension could be yet another.
treating the psoas is probably best done by gut smashing with a big ball like supernova, and couch stretch.