The only thing that has worked for me to keep my sleep regular is to go outside first thing in the morning and sit for 15-20 minutes. Rain or shine. Even if it's cold or cloudy.
This has been game changing and keeps me on a regular sleep routine.
Seconding that the time you get up, and light exposure then and during the day, has a massive impact. If you habitually sleep in late, then "going to bed early" does not work as you'll lack sleep pressure. It's putting the cart before the horse. You have to treat it the same way we do jet lag; when we travel to the other side of the world, we adjust by just gritting through the morning. Takes few days to shift.