I had DSPS since my teens, I could never sleep before 3-4 a.m.
...some doctor told me to sleep 2 hours later every night to reset the clock, and then I got this non-24h ever since... Before this, when I "only" had ...DSPS, I was still decently healthy and functional if I could sleep [late]... When I did the 2-hours-per night delay my brain and body broke down... When I sleep fine on the right hours for a few days, most of my ...problems go away by themselves. So we have to be very careful with what doctors say... this 2-hours-per day delay therapy. It is very, very dangerous.
I also have had delayed sleep phase for a long time, since anyone can remember. My body's preferred time has always been to go to sleep around 3 or 4 am, wake up around 10 or 11 (or 12 if I could!)
The reason I'm responding to your post is that I've never before heard that the 2-hours-per-night delay can really mess a person up, but I tried doing it on my own, a few times, about 10 years ago (after reading advice that it was the best thing to do), and it didn't work at all for me, and it did seem to screw up my rhythms even further.
...Not anywhere as badly as it affected you, though. I'm sorry to hear of your trevails.
As you say, when I can get on normal folks' typical schedule (sleeping from 10 or 11 at night to 6 or 7 in the morning), it only lasts for a day or two and then I'm right back into a strange rhythm, because it extends beyond the 24 hours and I can't easily stick to one spot.
I've tried light therapy, all the typical bedtime-sleepyness tips -- except for melatonin because I was always cautious about using a hormone on my own (and I mainly lived in places where it wasn't available without a prescription).
I've just received my 23andme genetic results and the results of many blood tests, and I'm about to self-treat my health in a number of ways, and will be interested to see if my sleep pattern changes as a result of these interventions.
Good luck to everyone here.