I recently heard someone say "people will keep the style from the phase in their life they felt most comfortable in", for example women who were teenagers in the 80s and still rock the poofy hair. I think Lily is doing something similar, but you can see it more in the way she behaves than how she styles herself.
To me it's obvious she wishes she could still live like the she did when she was a teenager, hence the neverending stories about her youth, on the podcast, in videos, etc. She seems to have had a carefree youth, spoiled as the youngest child, no committments and obligations. I feel like her family expected her to get married, buy a house, have kids and she did just that because her two sisters have done the same thing. I don't doubt she also wanted these things, but had a very unrealistic idea of how they would go.
She likes the IDEA of being married, but can't deal with someone being in close quarters to her 24/7.
She still fries the everliving daylights out of her hair because that's the way she did it in her teenage years and can't emotionally let go and adopt healthier habits.
She likes the IDEA of being a mother, but didn't expect all of the negatives that come with it and freaks out if Grey doesn't behave the way she wants. The incessant stories about her making a mess and toys being everywhere and even building huge shelves to erase every trace of a kid living there.
To be honest I'd be miserable too if the way of living that I've been taught is the best, the one and only doesn't actually turn out to be "the one". I think she slowly started to realize that when her daughter was born and it wasn't all daisies and sunshine and wishes herself back to when she wasn't feeling so miserable, like in her teenage years.
I know she can be insufferably moany but sometimes I understand why she acts the way she does, even if I absolutely don't feel sorry for her. I don't throw this around lightly as I have dealt with it myself, but I think talking to someone (therapist, councellor) could help her realize why she feels miserable and that it's not normal to be moody 24/7.
Tl;dr: I think Lily's incessant rambling about how great her youth was is a coping mechanism because the life she leads isn't what it was hyped up to be. I have seen it within my own family and it can not only make yourself miserable, but (take it from me) can fuck up your kids aswell and make them feel unwanted...