Date: 202103
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Operator: Fractional
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: Commercial Fixed Wing
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Troubleshooting; Training / Qualification; Time Pressure; Situational Awareness; Distraction
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Air Traffic Control; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Environment - Non Weather Related; Human Factors; Company Policy; Airspace Structure
Primary problem: Environment - Non Weather Related

Narrative: I was working Sector X (AOA [At Or Above] FL360) and another sector was being saturated (Y/Z/A combined. Training was being done at this sector); so they split off A to me. There was a lot of traffic and weather in the area. Apparently; restrictions were put in place that aircraft entering ZZZ1 [Center] from us and landing at ZZZ [Airport] needed to be AOB FL260. I was working an Ultra High Sector. When I was given the briefing by the trainee; the trainer ran over to my scope and said that Aircraft X needed to be AOB [At Or Below] FL260. The aircraft was at FL430 and maybe 40 miles from the boundary; on top of same direction and opposite direction traffic. Aside from that there were numerous conflicting aircraft issues already fixing to happen. My EDST [En-route Decision Support Tool] had a bunch of red alerts of which should have been fixed prior to the aircraft entering our area. Again; this was on the sector I was gaining; NOT the one I was already working. I was bit overwhelmed taking the sector just due to the amount of work that needed to be corrected due to the neglect of the training team.I descended Aircraft X to FL390; knowing I would have to move or quickly descend the aircraft through two others converging at FL390 and another directly below at FL380.I assigned the altitude and realized the speed gain was great enough that I could get Aircraft X down to FL360 (base of my stratum) and miss Aircraft Y; heading the same direction at FL380. To ensure I had room and miss opposite direction traffic at FL390; I turned Aircraft X. My turn was a bit too aggressive and in doing so; the wind caused the aircraft to lose a bunch of speed; resulting in the CA going off. I turned Aircraft Y to the left to keep separation; but it was already lost. From my observation; I showed a loss of separation to where they were separated by 4.9 miles and 900 feet vs the 1000/5 I need. I didn't stare too long as I had to go on to other separation tasks that were not completed prior to receiving the additional airspace/traffic.First; splitting sectors at the last minute has become all too common an occurrence with this COVID schedule and working 'more with less.' The whole environment is unsafe and the things we do are dangerous; resulting in issues like the one I had.Secondly; I won't be taking sectors from a person who doesn't have it all figured out; no matter what the Supervisor says.I take pride in what I do and having separation errors is unacceptable. I had the pilot helping me out the best he could and I second guessed myself that the aircraft would descend through the altitudes fast enough in a straight line Aircraft X started to outrun Aircraft Y as he descended. My turn was too aggressive; and I never played it into my head how much the wind would impact the turn. I think just trying to take action to descend the aircraft to meet a restriction overruled my common sense to think how my actions would play out. Solely my fault; but it could have and should have been avoided had other things been done in a timely manner prior to my taking the airspace. It seems to be a trend to do everything last minute; which is what creates such mistakes. Had I worked that aircraft and known of the restriction; Aircraft X would have descended below all my traffic 100 miles prior. Maybe not to FL260 so as to penalize the aircraft; but at least below those who were deviating and being rerouted around the weather.




Synopsis: Center Controller reported having a loss of separation after assuming a busy sector from a training team.