ZFW controller experienced a loss of separation event when failing to hear an incorrect altitude assignment read back; listing distractions and read back discipline as contributing factors.

Narrative

Aircraft X was assigned 16;000 to remain above Aircraft Y on a somewhat crossing reciprocal course. Aircraft X readback 10000; which I missed; I was somewhat distracted by an issue with FSI not complying with their Letter of Agreement and two aircraft I was attempting to enter flight plans on for the next portion of their flights. I noticed Aircraft X descending through 16000; while obliquely converging with Aircraft Y and about 7 miles away from Aircraft Y; and questioned his altitude...I then turned both aircraft 40 degrees left immediately. Separation was probably lost for a short time; and as the aircraft were not a threat to converge; I simply had Aircraft X stop his descent to stabilize the situation; he leveled at 14800. The aircraft; by this time; had already become separated. Recommendation; request assistance from the FLM (Front Line Manager) on the flight plan matter; it would have been useful training for a developmental. Focus more directly on the read backs; an area I usually pride myself on; but one that I apparently slipped on today.