MAF ATCS reports minor loss of separation due to controller technique; situational awareness; and possibly fatigue.

Narrative

Aircraft 1 was northeastbound climbing to 9;000 MSL. Aircraft 2 was inbound from the northeast at 8;000 MSL per the LOA with ZFW. After taking the automated handoff from ZFW; I intended to descend Aircraft 2 to 6;000 MSL on his initial call to be well below Aircraft 1 but was distracted working 3 T38s; 1 T45; and 2 additional aircraft doing practice approaches at MAF. Additionally I had an E145; and another VFR aircraft inbound to MAF for a full stop as well as a VFR departure off MAF to the southwest. I 'rogered' Aircraft 2 on his initial call to take care of other approach clearances and neglected to go back to Aircraft 2 for descent. I scanned back to the northeast to see Aircraft 1 and Aircraft 2 on converging courses approximately 2.8-3NM apart. Aircraft 1 was climbing through 8;800 MSL and Aircraft 2 was still level at 8;000 MSL. I immediately turned Aircraft 2 right to a heading of 330 and turned Aircraft 1 right to a heading of 180. I then started issuing the traffic to Aircraft 1 who reported Aircraft 2 in sight. Aircraft 1 was instructed to maintain visual separation and was turned back on course. Aircraft 2 was advised that Aircraft 1 had them in sight and was maintaining visual. The two aircraft carried on without further incident.This event occurred after several hours of OJT on arrival/departure in the first 6 hours of my shift. I feel that mental fatigue along with the volume and complexity of the traffic at the time contributed. I still needed to ensure that my separation was accomplished before being concerned with approach clearances. I should have maintained better situational awareness to ensure that the required separation existed before tending to other duties. I feel that better situational awareness as well as more time to 'digest' previous training sessions will prevent this from happening again