Tough to tell from the photos, but your saddle looks too far forward to me. I'd start with pushing the saddle back and lowering it. Next, I'd consider flipping the stem up.

Really need better photos without your head ducked like that which makes everything look weird. Also, have you tried rolling your hips forward instead of sitting so flat on the saddle? That would relieve some of your back pain, but you may have to adjust the angle of your saddle for crotch comfort.