I also choose a keto type diet. I've lost 90 lbs over the past year and a half. I think it's important to not shun all fruits and vegetables. There are low sugar fruits (bell peppers, tomato, cucumber, olives) and there are TONS of low sugar vegetables.
Weight loss does affect hormones, and so I think it's reasonable to expect it could affect mood both positively and negatively. I think if you're seeing huge negative swings, something needs to change. Perhaps slow down the weight loss, supplement strategically, add back in keto-friendly fruits and vegetables. Journaling, walking, yoga, pets, etc. can all help with mood.
Check out Trudy Scott's
blog, she does a lot of articles on GABA in particular.