Incomplete evacuation is ruining my life (help)

(I have PT soon, just struggling in the meantime)

I already use a stool to put my feet on when I go for a poop. I do deep diaphragmatic breaths.

But my body is just working against me, right at the end when I release for the first time, my anal sphincter closes up and refuses to relax. I do not know what else to do. I feel miserable for the rest of the day until I go for a second time that day.