I think she was very unhappy during her pregnancy and postpartum. They got pregnant too quickly (probably because of her age) - she had no time to really get to know the ins and outs of the job, no time to just enjoy the change of status before everything changing with a baby.
I bet she'd always imagined a very different type of pregnancy - lots of shopping and hanging out with friends, as a celebrity probably planning a Hello-type photoshoot, lots of freebies sent from companies and fashion houses for her and the baby. A California celebrity type of pregnancy.
Very different from what she got - lots of protocol and rules, horrible press from the tabloids picking at her, no freebies, all her friends in other countries, work engagements mostly boring and stuffy UK things, royal wives (at least the senior ones) expected to disappear and be discrete for maternity leave and after the birth. She wanted to do exclusive interviews with Oprah and Gayle on the birth and on the first year of marriage and was told no.
In addition she and Harry had experienced all those amazing crowds turning up for them wherever they went in the beginning and this is where as an American and a minor celebrity prior to marriage she mistook all that cheering and goodwill for the popularity of Harry and her alone, whereas the crowds come for both the royal family and the novelty of a new glamourous member. And I think her American publicists got this wrong too. They all assumed that Harry and Meghan would be able to take that popularity and use it for commercial gain, and the royal family needed them so much that they would agree to the half in half out royalty model that they proposed in their exit statement.
I think they still could have translated what they had to work with into a decent life in America if they had accepted that they weren't going to be able to have the half-royal life they asked for - but it seems like they are BOTH still so angry at being told no to that, that they can't stop this complaining and moaning cycle.
It's turning into a very toxic dynamic - which is very car-crash-worthy for all us rubberneckers but not really doing any of the principles involved any good.