When I get unmotivated I usually do a couple dozen,sometimes up to a hundred/twohundred at a time, and leave it again. At some point I will get the motivation to do all at once. Last time though, what really helped me, was installing the SRS Reorder Script. This way I could get the burn items out of the way, while not having the apprentice items coming back all the time (the way I see it, they’re of timer anyway, so was always going to fail a bunch). This got me through the pile right quick (~600 at the most) as I wasn’t failing as much, about 60-70%, being more motivated as a result.
(You do need to tinker the settings in the script a little bit, see script comment section)