It's being repeated all over this comment section, but "Bury your head in the sand" isn't really an answer. Yes, the news media and social media both highlight the worst of things, and they profit from fear mongering, but in times of genuine crisis and breakdown not staying informed is terrible advice.
It can be argued that news can still be obtained elsewhere and I'd agree, but that isn't the advice I've been seeing here. It's pretty much just "walk away from news and social media" without any mention to where to go instead or some sort of platitude about how "things aren't really that bad".
Ignorance isn't going to fix anything, or help people prepare for what's coming or come to terms with what's happening now.
Things right now are not great. It's not just because of doom and gloom media (plenty of things are objectively messed up, no yellow journalism necessary), it's not just because of the internet allowing us to be more aware of the issues (the internet isn't exactly new at this point, and a lack of awareness about genuine problems should concern us far more).
Your stress is not abnormal or unreasonable. We should all be feeling more stressed out right now, either because of threats to our freedoms or our democracies, or because the effects of climate change are becoming harder to ignore, or because our children are being killed in their classrooms, or because disease is spreading, or because prices have skyrocketed, or because of acts of cruelty going on around us, or any of the other problems which are developing and/or worsening.
I'll propose an alternative: Keep paying attention. In fact, if there's something in particular that really worries you then really pay attention. Learn all you can about that issue. Consider what kinds of things might improve the situation, evaluate the extent to which there are things you personally can do help improve them and to better prepare yourself if they don't improve. If you find them, take those steps. Be proactive.
If there's really nothing you can do, or you've already done all you can then keep an eye on the situation for changes and opportunities, but try to come to terms with things outside of your control because either way we're stuck with them. Instead focus your efforts on the next thing that troubles you.
Climate change isn't something you can solve on your own, but you can stay informed and prepare. School shootings aren't something you can solve, but you can vote and campaign for gun control legislation and if you have children you can take steps to help them prepare. You'll likely have to expand your search for ways to help beyond start ups. Startups are about making money, any "making the world a better place" is incidental.
I would recommend getting out of marketing. You seem to recognize that you're in a toxic industry that's hurting people. Let it go. You've got skills that transfer. Maybe you can't find work saving the world, but there are plenty of jobs that don't involve rubbing salt in our collective wounds.
Also, consider talking to a therapist because while it won't help the underlying stressors it can help you find ways to productively deal with living under the stressful times we're experiencing, and if nothing else it's one more proactive thing you can do. All that stress and anxiety is your body telling you that there is a problem and that you should act. It's telling you to make a move and to pay attention. Doing those things is far better than trying to ignore the problem and failing (because you aren't blind) and ending up stuck feeling helplessness and despair.
Finally, take care of yourself. You'll be in a better position to handle whatever the future brings the healthier you are. Stress and depression are terrible for your physical health as well as your mental state. Treat your body better if you can.