The comedian Patton Oswald has a (NSFW) bit about going off his anti-depressants because he was afraid he couldn’t get them after civilization collapsed. While entertaining, his observation about medications not being available is one that I believe is likely to be true.
Modern medicines are miracles of technology, and there is nothing readily available to replace them if they are gone. I wish that there were an easy button, some sort of “collect dandelion flowers, dry them, and make a tea year round to treat depression!” level of answer to this, but there isn’t. Thankfully, multi-year collapse scenarios are rare, and global collapse is also very unlikely, so the odds are good that having a stockpile of necessary medications is going to be enough to get you through the more likely “Hurricane Katrina” style scenarios.
But if it isn’t short term, then what does that mean? Different people have different ideas about what a global collapse would look like, but using what has happened in the last 50 years as a model for prepping, I don’t believe that a full-on “face colander Mad Max”-style collapse is likely. Far more likely is the long, slow slide of Rome where things just get worn down, the economy contracts, and systems just aren’t as resilient... like the water system in Flint, Michigan, but played out on a global scale. So I believe that any collapse is going to be more of a "turn back the clock to the 1910-1930s" rather than "turn back the clock to the Stone Age". I think it is important to state this up front, because what I write here is based only on my own ideas, and not some sort of group consensus.
Truly high-tech pieces of civilization rely on clean rooms and high purity raw material inputs. It would be impossible to make an integrated circuit (a computer chip) using the technology available even 100 years ago, and modern medicine requires the same purity of ingredients, very precise reactions, and high-tech testing to ensure that the end product is actual medicine instead of poison. This means that if there is a collapse that disrupts the economy, computer chip and medicine fabricators will be the first to experience disruption in operations. This isn’t in itself going to end the world, but what it means is that as the digital things wear out, they won’t be replaced by better cheaper digital things and that once the current medicinal stocks are on hand are used up, people will begin to look for replacements.
If you think about all the “cure-alls” sold in the early 20th century, where doses of cocaine or morphine were included in products sold over the counter, it probably doesn’t inspire much confidence in meaningful treatment methods from the scientific knowledge of the era.
The even worse news is that there aren’t any 1910-era replacements for selective serotonin reuptake inhibitors (SSRI) or mono-amine oxidase inhibitors (MAOIs) that are routinely used to treat things like depression and anxiety. I recommend a huge dose of skepticism when listening to anyone who says that you can treat depression with common herbs you can grow at home. Unless you are talking about cannabis (which does have some medicinal uses, which is why it's regulated as a drug), the ability to grow medicinally useful compounds at home is very small, and those that you can grow are not going to be useful for mental health.
There is some research that shows good effects from saffron and turmeric in treating depression. Turmeric grows in warm wet parts of Southeast Asia and saffron can grow anywhere from Spain to Afghanistan, but the three stamen per flower make it incredibly hard to grow in quantity. If international trade is disrupted, getting these in quantity might become difficult. If you are a reader who lives where turmeric is grown, that might be something to look into. If not, perhaps international trade will make turmeric available; after all, spices were a huge driver of international trade for the bulk of recorded human history.
But even though these spices were available a hundred years ago, looking back to the early 20th century it becomes clear that a lack of effective mental health treatment options was one of the foundations for the rampant alcoholism that eventually gave us Prohibition, which gave us a huge increase in organized crime, which eventually ended Prohibition. Self-medication with alcohol to treat depression or anxiety (think about how gin and whiskey took on the names “liquid courage”) is something that you and your tribe will want to avoid. Alcohol has many uses from sterilization to food preservation and should be part of your preps, but you must be cognizant of the dangers of alcohol addiction in a world without modern medicines.
Then there's marijuana. Despite a near total Federal prohibition on it, a somewhat thriving underground economy supporting an obvious demand for the stuff has managed to stay in business. Several states are legalizing marijuana, and so far it hasn’t been a big deal and no one really cares. Marijuana doesn’t cure cancer, nor has any health benefits to a healthy human being, but it may be useful if someone in your tribe has crippling anxiety and/or depression. However, the long-term side-effects of heavy marijuana usage would make this an option of last resort in a long term collapse scenario.
Unfortunately that’s pretty much it for the list of chemicals or plant products that are available to the 1910 level of technology. If I haven’t made the case by now that there just aren’t any good “low-tech, resilient” chemical solutions to treating depression and anxiety, it isn’t for lack of trying on my part. The dangers of addiction through self-medication are real, and the “sanitariums” of the era were little better than poorly run torture prisons to keep the mentally unwell out of the public eye.
The Talking Cure
The last option is the one that I support. Taking medications as a therapy under the instructions of an actual doctor is recommended when available, but there's almost always someone available to listen. Having people feel comfortable enough to talk about their depression is far harder to achieve than it sounds -- people don’t want to look like unproductive parasites in a survival situation -- but it's important to create some sort of tribal culture, service, or space where it's acceptable to do that. A resilient tribe has always had priests, pastors, therapists, and elders, to whom people could go for advice. Those people may not always have had good advice to give, but simply listening is a service that isn’t going to go out of style any time soon.
Above All, Plan
Like I wrote at the beginning, there aren’t any easy buttons for this issue, but mental health and its evil twin addiction are not going away just because civilization took a hit to the jaw.
Plan for medicines to become scarce, or not available, and that more and more members of society at large will fall into addiction.
Plan for increasing your security posture to a level where you aren’t rolled over by an addict looking for their next fix.
Plan for creating the support structure you need to deal with depression and anxiety. The support group is a great way to go in terms of a solution that can be implemented by people without requiring outside expertise (think of the Alcoholics Anonymous model if your tribe is large enough).