I have always had delayed sleep phase disorder (self-diagnosis)
Melatonin no longer works: after some occasional use, it now only works in high doses;
Melatonin isn't a sleeping pill* and you're probably not using it correctly.
The correct** way to use melatonin is in low doses 5~8 hours before bed time.
Melatonin is your brain's way of saying "hey, it's dark out, let's push over the chemical dominoes that will lead to sleep."
On the other end, when you wake up, you either need a light box or some sunshine.
(Ideally you want a light source of at least 10,000 lumens at 480nm (IIRC))
This sets off another chain of chemical reactions that let your brain know that it's time to wake up.
As a result, it prevents your sleep phase from shifting forward by however much your delay is.
*Most of the usage directions you find online are wrong.
**I'm not a sleep doctor but I talked to one recently, you should too.