I am going to try and answer this with my limited experience. Disclaimer I am just a senior phd candidate myself.
The question you are asking is by no means a trivial one nor are you the only one wondering about it. Every single phd student, in almost any field, that preceded us and that will succeed us, has/will wonder the same. So, as a first piece of advice: If you feel lost, you are not alone!
My academic journey has lead me to believe a somewhat unpopular opinion; an experienced advisor can be sufficient but is not a necessary ingredient in this process. Surely, having someone to gradually introduce you to an area with questions and tasks of increasing difficulty, and provide sufficient support and guidance throughout the process can help come up with your own questions. Of course, in most cases that is outside of our control.
Although, I do believe it's not necessary. Some of the first original ideas and results I came up with were a product of discussing problems and concepts with other phd students in my group. Bouncing ideas back and forth, attending conferences all together and discussing the presentations and results, having weekly "Theory Seminars", etc. To me, the group of your fellow phd students is almost as important as the advisors themselves. But again, this is a parameter of your environment and not necessarily in your control.
So, what is in your control? I think the biggest value-for-time if you are stuck in the no-mans land, is to read. From the consensus most influential papers of your area to surveys and recent results. Any chance you get, keep building on to that picture in your head of what your area looks like; what are the biggest challenges, what are the most valuable tools, who are the key people whose work you need to follow. It's a slow and tedious process and you have to read a lot of papers, sometimes to even identify the influential ones, but do it. Make sure you read the papers at your own pace, but as your peers have advised you, make sure you understand them (yes, that includes - by definition - the theorems and proofs). Once you have read enough papers and painted your picture ornate, you will start noticing the small, and as you go along larger, "holes" that you can slowly fill with your questions and hopefully answers.
Finally, as anyone who has written any paper ever will tell you, you don't just magically start writing a paper. The whole process is a product of a lot of reading, a lot of trial and error, and a lot of writing and editing. I too have felt at times overwhelmed when reading a paper and truly wondered how, or even if, I would ever be able to come up with a result of the same quality, but don't think about it that way. Start small and keep going. Just remember that when you are reading a paper and it feels daunting you are consuming, in a few minutes, work that was a product of probably hundreds and hundreds of hours.
I am sorry I don't know much about your field, so I can't provide specific advice, but these are my thoughts. Please take them with a grain of salt, as I believe we are on the same boat. Good luck!
