I'm in my 40's and grew up during the War on Drugs. I've done plenty of drugs and known many people who did plenty of drugs. I've been well positioned to compare what I've been told to what I've experienced. I'm extremely skeptical of most cautionary drug tales.
Admittedly, it isn't always good to consume things that cause psychotic states on top of existing psychotic condition.
There is very little evidence (no real scientific evidence) that hallucinogenic drugs increase the chance of developing mental illness. Scientific evidence seems to indicate that psycadellic drug users have an incidence of mental illness that is the same as everyone else. No link found between psychedelics and psychosis-Nature.
Here are two excerpts from the papers that the Nature summarizing
We failed to find any associations between lifetime use of psychedelics and past year serious psychological distress, receiving or needing mental health treatment, depression, anxiety, or suicidal thoughts or behavior in the past year. Rather, lifetime use of psychedelics was associated with decreased inpatient psychiatric treatment.
Consistent with hypotheses, lifetime classic psychedelic use was associated with a 19% reduced likelihood of past month psychological distress, a 14% reduced likelihood of past year suicidal thinking, a 29% reduced likelihood of past year suicidal planning, and a 36% reduced likelihood of past year suicide attempt. […] By contrast, lifetime illicit use of all other substances was by and large associated with an increased likelihood of psychological distress and suicidality at or above the trend level.
Most people start experimenting with drugs right around the same age that they also start manifesting serious mental illness. coincidence an look an awful lot like causation.
Maybe we'll find out that these studies got something seriously wrong or maybe we won't. There is a growing body of research that shows hallucinogenic drugs can alleviate the effects of depression and PTSD.
Society has spent the the last four decades spreading misinformation on these substances. The potential dangers and benefits of their use warrant through unbiased scientific research. I am not sure if they can cause or worsen mental illness in the long term or if they have long lasting benefits for most users but my own experience is that they can be beneficial and are for the most part harmless. I've known people who abused them to their detriment but I think that as a class of substances that can be abused they aren't all that different from many other things.
As far as having a sitter when you trip, I always hear how necessary it is but don't care for it at all. During a trip of "self realization" I find it interrupts my thoughts and interferes with getting work done. During a just having fun trip I find sitters a fussy annoyance. I've known people that require a sitter for their own safety and sanity. User experiences will differ but you'll probably be fine anyway you go if aren't a wing nut at the start of it. Going into a trip with a positive open mindset will do more to promote a positive experience than anything else you can do including having a sitter.