Enroute Controller described a loss of separation event during exiting holding procedures; the reporter listing workload; failure to request a D-Side and attentiveness to altitude assignments as causal factors.

Narrative

I was working an inbound rush into ZZZ Airport at a low sector over ABC. We went into metering and the airplanes needed to be put into a holding pattern. Military Airspace was also active so I had to keep the holding stack at FL190 and above. I took as many airplanes as I could from high altitude which had saturated all my altitudes. I had started to clear airplanes from the stack on course. I had 2 going in at the time and started stepping airplanes down to receive more airplanes from the high sector. I saw one airplane leave FL 205 for FL200 so I looked for the next plane to take down. I saw Aircraft X at FL230 so I descended them to FL210. I didn't see Aircraft Y at FL220. I noticed it very soon after I gave the descent and told Aircraft X to climb back up to FL230. I lost separation before Aircraft X could regain FL230. I will in the future pay even more attention to altitudes before I issue one to an aircraft; especially in a holding stack. I could have also asked for a D-Side to help me scan the various altitudes. I could have taken less airplanes from high to allow myself more altitudes to work with.