Center Controller reported a loss of separation between two aircraft.

Narrative

Working a busy push; I had two aircraft going against my flow of traffic; north to south. One needed to go down and the other was climbing; with several departing aircraft going east to west. I was focused on getting one down that I missed how slow the southbound guy was climbing. He was climbing to FL200; Aircraft X; and was out of FL193 when I noticed the westbound guy I had stopped at FL190; Aircraft Y; was getting close. I have [a] 3-mile separation below FL230 in that area and never lost the 3 miles but it was close. I turned Aircraft Y 20 to the right to ensure the 3-mile separation was maintained and told Aircraft X to expedite to FL200. I called traffic to Aircraft Y and he called him in sight and once I had increased separation; I turned Aircraft Y back on course. Aircraft Y was right on the bubble and Aircraft X quickly leveled at FL200; but it was too close for comfort. I was working two radar positions for the first time today in over a week and a half. I am only certified on these two positions and they are rarely split out. With the current training suspension due to COVID; I get to actually work traffic one or two days a week if I am lucky. I am not proficient at all and I usually get called to work busy sessions because that is when it gets split out with the two sectors I currently do not have certifications on. I rarely get to work and I haven't been able to train in a month and a half; yet; I am expected to stay proficient and come and work busy sessions due to COVID protocols. I think the FAA as a whole needs to look at the strain the COVID protocols are putting in place for different facilities and try something new. It is incredibly difficult for someone to rarely work Position and then pick up and work busy traffic. We should be restarting training immediately and be better about splitting sectors out to prevent this. Working Position one or two days a week isn't enough.