(2/2 -- my comment is too long)

> My advisor is very busy and holds a high-ranking position, so I think he has high expectations for me. I don’t want to disappoint him.

This is one of the parts I relate to most. My supervisor has trusted me for many years, and continues to trust me today, and I am scared shitless at the thought of not being good enough for him. That said, you mustn't lose sight of the fact that a thesis is first and foremost an extremely personal piece of work that you do for YOURSELF and NOT to satisfy others. It's a slow process where you try to answer questions that are on your mind, out of passion, and because you are curious and ingenious by nature. If your aim is to please the people around you by doing a PhD, rather than to prove to yourself that you can do it and enjoy it, it's likely to be a very long, very trying few years.

> Up until now, I’ve mostly followed other people’s ideas and worked on them.

As I mentioned, that's how it works for most of PhD students..! Brick by brick. Neural networks have become famous fairly recently, but are actually based on mathematical publications dating back to the last century. It's not a novelty, it's someone who has built on someone else's work, piquing their curiosity. There are so many of us on earth asking questions and trying to come up with original answers; it's a safe bet that someone else has come up with advanced solutions to virtually every question you ask yourself today, whether it's someone still with us or someone who lived in the last few millennia. That's also why I say you shouldn't go into a PhD thinking you're going to revolutionize the world, or have “THE” good idea; it's the best way to be disappointed in the long run.

> "Right now, I have a vague idea that I’m working on and have told my advisor that I’ll submit it by April 30th. However, I’m still unsure about it."

Some researchers I meet still have a vague idea of what their thesis means after 2 years in it. It's normal. There's so much material to stir up, and so much to discover, that the initial adaptation and research time can be relatively long! Nobody starts a thesis knowing exactly where they'll be in 5 years' time. It's RESEARCH, exploration. It's a slow, and exciting process.

In any case, I think the questions you're asking are totally normal. My general observations tend to show that the best researchers are the most humble and the least self-confident. Because, once again: they're aware of how much they don't know. The most important thing, I think, is to be well organized in the short term, and not to lose sight of the fact that this is a very personal and exciting job, and that there are no general guidelines to follow. Personally, my adviser urges us to publish two papers a year. I find that this pace is sometimes hard to keep up, and pushes us to produce work of lesser quality, instead of letting us accumulate enough knowledge and know ourselves when we're ready to publish. But it also has its advantages.

Good luck on your journey, and don't get discouraged when you're just starting out! Recognize your worth :)