Doing a research degree is a completely different experience from doing a taught degree. There’s no one standing over your shoulder saying “do this, do that”. Your supervisors are there to provide guidance, but your research and progress are ultimately your responsibility. As a result, doing a PhD requires substantially more self-motivation and self-discipline than doing an undergraduate or master’s degree.
My advice to a future PhD student would be to establish a routine that works for you as early as you can, and try your best to stick to it. Treating your PhD like a day job helps you stay on track with your progression. Yes, you’re still a student (enjoy those student discounts!), but don’t let the scheduling flexibility afforded by your student status hold you back from getting things done.