vagfilm
A couple of comments about memory fading, so that you have some peace of mind…
Firstly: you are still at least 20-25 years away from having any form of memory loss, and the majority of studies show that habitual memory training (such as music memorization) helps in delaying the onset of memory loss of that particular type of memory…
Secondly: Memory loss is usually event arbitrary and category stable, ie, you tend to forget the same kind of things (either names, or familiar objects, unfamiliar objects, locations, musical memories vs word memories…), but within a particular category the forgotten items are random. You say that you have consistent problems with one particular portion of one particular piece. That is a sign of poor memory formation not memory loss.
Third: forgot the third… I am over fifty after all…
Bottomline: relax because the problem lies in the piece not in you.
There is only one proven solution: sleep. And then sleep some more. If it does not cause you excessive stress, you may want to briefly practice that section just before going to bed.
Finally, you may want to set a deadline for commiting to that piece. By that deadline you either decide that it is a go because you are confortable, or scrap it because you are afraid of having a blank during the recital. There is too much at stake on this recital, as you acknowledge, and you may want to play it safe (no pun intended).