I'm not going to say exactly when but I had an existential crisis and a mental breakdown when I fully grasped the eco-socio-political reality of climate change and how we would respond, what that would achieve, and the fact that one way of addressing climate change would likely have a domino effect of negative consequences that would impact the environment negatively in other ways.

One simple example of this would be heatwaves causing people to use more air-conditioning and personal transport, driving up energy consumption and heat generated locally. A more complex example would be crop failures leading to more intensive agricultural efforts elsewhere and reliance upon the seas for food, leading to hastening the environmental collapse in those domains. A global example would be the pinball effect of destablisation of countries, the disruption to supply chains, the destabilisation of nearby countries, thus impeding global efforts for cooperation and climate change mitigation; there is no environmental policy in a state which has collapsed.

This happened a long time ago now, not like "before Greta Thunberg was speaking" but more like "before Greta Thunberg was speaking", and I've never truly recovered from it.