I completely empathize that people can suffer with depression for no apparent reason other than thats how their brain works, but i've noticed that Katie has been on a slight downward trend ever since she moved out of the house with her friends.
I think Katie is dissatisfied with her career because she does not perceive herself to be successful, and I kind of agree with this. Although she lives a privileged life (lots of freebies) I don't think by influencer standards she is 'big'. Her channel hasn't grown much and I think she has reliazed that she is not successful at the things she is passionate about (I don't think she stopped her true crime series because she couldn't be bothered, but because she was disheartened by the views compared to her other videos).
I think most can agree that people work well under a bit of pressure. When she was single, she had to rely on her own income to support herself and thus there was some pressure to work and get content out. I do think G makes enough money to support them - i've always had the impression that he paid 100% of the rent at their flat, as Katie doesnt earn a 'set' amount each month. I think the fact that she did not have to worry about supporting herself has taken the pressure off and she has lost all motivation. She has even admitted that she is dissatisfied with her content and is not motivated to make any.
I think this combination is whats causes her to feel like she is in a rut and has probably triggered some depression, but she does not acknowledge that her lifestyle is causing it because she doesnt want to give up the easiness/flexibility/perks of the job.
NB - this is obviously just speculative observation