Yes, too much sun for a true moss. Sagina subulata (Scotch or Irish moss, depending on coloring) likes full sun but it also prefers an organically rich soil with constant moisture and good drainage. Die out usually indicates either it's not getting enough moisture or too much (i.e. poor drainage). Other possible choices include Cotula squalida (syn. Leptinella), which is about the cushiest GC I can think of for bare feet :-) or Scleranthus biflorus, Herniaria glabra or wooly thyme. Wooly thyme flowers easily and thyme flowers attract bees in abundance, so be cautious when treading with bare feet during flowering season.

With any of the very mat-like spreading groundcovers - like all mentioned above - you increase the distribution and therefore a more rapid spread if you cut the pots into chunks......usually 4 parts for a 3.5" pot. It will take time to get full coverage. Like any other type of plant, goundcovers need time to establish before they start covering up a lot of real estate, generally a couple of seasons. And some varieties tend to spread better/faster than others. IMO, cotula is one of the better choices for this type of situation. You will find differing cultural info on this plant, but in my zone 8+ PNW garden, it tolerates full sun well, is quite tolerant when established, not too fussy about soils but will establish faster in a richer rather leaner soil and like good drainage. And it is fully evergreen (or everblack, depending on selection :-))

leptinella, I looked at some pictures and it looks nice.

The black looks awesome, and it seems like it'd be comfy for my feet. I think I'll try some of that and maybe a second dose of the irish moss, maybe if I put some more compost down under it that will help it all get going faster.

thanks for the help, it's good too because you're in the same kind of zone as me (I am in oregon, by the way)