I know those feels, sadly at the end of the day it can't be helped so I power through it but I always try to keep it as close as possible not sure why it is not that big a loss lol, I guess it is just the good feeling of making every bit of xp count that give you that warm fuzzy feeling.
I am in the same boat as you except I have thrown 3 rainbow spirits at that last SU and have just let it be now, if I get any more copies of her and they give the SU well then wahoo! But I wont lose sleep over not getting 10/10 Skill, 9/10 is close enough without wasting precious resources that could be better spent on rarer units.