I've been having a bit of an existential crisis about this recently. Is sustainability even possible anymore? We are obviously living beyond the capacity of the earth to cope right now. But is it even possible to sustain this many people (at current standards of living with foreseeable technology)?
If anything, the thing we need to be concerned about is being ready for changes, which will happen. We might need to grow different sorts of foods, focus on better insulation for our homes or move underground or underwater. We may need new laws to avoid wasting resources. But, there is no reason to be depressed about it. Those things will happen with time.