I think this is a real struggle. I've felt massively depressed by the state of things in the past. I don't have a silver bullet, but some stuff has really helped me:
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Do something about it! Even if it's as small as talking to your friends, voting on green issues and signing petitions. Positioning yourself on the side of fighting against environmental damage, is a much happier place than watching it happen fron the sidelines.
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Remember the environment in the big and the small. As in, CO2 needs to be reduced, we should fight for that, but also, bringing more environmental stewardship in your life, whether joining a nature reserve conservation group, or just taking care of your houseplants and garden.
It might feel devastating sometimes to be witnessing the loss and danger that's happening. But the people mist perpatuating climat change are the one's who don't even understand what we're loosing.
Ronald Regan once said "a tree is a tree is a tree" to justify deforestation. What an idiot! Trees are beautiful and amazing and individual. I would much rather feel anxious than blind to the incredible beauty that exists in our world.