Isolation

One of the major mental health problems preppers need to consider is the effects of prolonged isolation on their team and tribe. Humans are generally social animals, and so we do best when we have contact with other people and can work together towards a common goal. Being cut off from social interaction affects different people in different ways; some can handle it, while others will reach some level of insanity rather quickly. 

With the current “quarantine” guidelines and social distancing being pushed harder every day, we're getting a taste of what life could be like after a major SHTF event:

• A significant EMP attack or CME causing the loss of our electrical grid would drop civilization back to roughly 1900s levels of transportation and communication.
• A real pandemic that kills 10-20% of the population would have much of the same effect, as infrastructure would begin to fail due to lack of maintenance.
• A truly rogue government imposing strict controls over the population would come close, but there would be (and is) resistance to alleviate some of the isolation.
• People getting stranded in their cars during blizzards is an annual event up north. Some of them make it out with a good story to tell, while others don't survive the experience.
• There were Japanese soldiers that refused to surrender at the end of WW2, the last one spent almost 30years living alone in the jungles of the Philippines before finally accepting the end of the war in 1974.

Looking back through history, I can find many instances of people being isolated for long periods of time, individually and in small groups. How they dealt with the lack of contact with others, and the reactions of some of them, might provide us with clues on how to prepare for something that could happen.

Biosphere 2

Briefly, Biosphere 2 was a sealed complex that  housed 8 people for 2 years. Completely separated from the outside world except for the windows, they had to grow their own food and keep enough plants alive to provide the oxygen they needed to breathe. They had mixed results: extra food had to be brought in and the oxygen levels dropped to dangerous levels, but all 8 people survived. 

About half-way through the experiment, the crew had split into factions which is a common occurrence in isolated groups and is something to watch for.

This is an extreme case, and is closer to setting up camp on the moon or Mars than anything we will see post-SHTF short of a total nuclear war and the destruction of a significant portion of Earth's ecology.

Submarine Duty

I've known several men who served on submarine. They had to pass some pretty thorough psychological testing to qualify for that duty; the Navy has probably the best understanding of how people will react to prolonged isolation of any organization on the planet. The military discipline and sense of duty each individual has keeps them fairly sane, but there are still problems that pop up when you're spending months under water. Good food (the subs get better rations than any other group in the Navy), scheduled releases of entertainment (so there is always something new to look forward to), and the amenities provided by having a nuclear reactor on-board (plentiful fresh water and electricity) help keep the worst of the boredom and sense of deprivation to a minimum.

Space Flight

We have a very limited history of space flight, but sticking three men into a capsule the size of a modern SUV for a 8 day trip to the moon and back 50 years ago is a good example of isolation. Early flights were crewed by military pilots, so once again their discipline and sense of duty had a lot to do with the success of the missions.

The few space stations that humans have managed to put into orbit have rotating crews and regular supply deliveries. Crew members have work to do, and staying busy wards off the feelings of loneliness. They also have excellent communication with people on the ground, which helps minimize the feeling of being cut off from society.

Antarctica

There are scientific stations in Antarctica that are staffed year-round. “Wintering over” is being there for the 6 months where transportation is not available due to the extreme weather conditions, and it is about what you'd expect to experience if you crawled into a bunker for a couple of months: tight quarters, limited communications, no resupply, no escape, and no way to survive outside the buildings for more than a few minutes. 

The physical and mental effects have their own medical term, “winter-over syndrome”. Irritability, absent-mindedness, aggression, and insomnia are common symptoms -- think of it as a prolonged period of living with a three-year-old. Gossip and rumors tend to be one of the worst aspects of life in these situations, creating tension and mistrust among the inhabitants, so keep lines of interpersonal communications open and stomp on gossip as best you can. 

A psychological view of the stresses can be found here, but it's a hard read unless you're familiar with the basics of behavioral science. Most such articles are locked behind pay-walls, but I did find that one freely available.

The biggest aids to avoiding problems that I have seen are a common history, a set of goals to achieve (stay busy), and good communications, and new information or entertainment and giving people a sense of privacy go a long way towards keeping them sane. That said, being locked up with a bunch of quarreling idiots is a good way for some of them not to survive the experience.