Sarah often mentions being on holiday as a child with grandparents and how happy she was, I think for whatever reason she was deeply unhappy in her day to day life, she frequently mentions how they didn't have much money. She associates happiness with holidays, someone else has mentioned it but lots of what she does is for that little girl inside her- 100s of Christmas presents and birthday presents, every second thing being a surprise, constantly buying clothes and random other things and constant holidays. It's not for the children at all, it's blantantly obvious she has next to none maternal skills its all for that little girl. It explains why she can't stand Isla she reminds her far to much of herself and why she stayed with creepy when anyone else would have ran a mile because he's the one who gave her money and who she believed she needed to keep getting money to fuel all the above things. She full believes money equals happiness but it's clear to see just how miserable she actually is.