In a PhD there is no set syllabus to follow, so you have to decide for yourself what you're going to do. Since most people don't know — when they first start — what the required standard is, the natural temptation is to aim as high as possible with the most ambitious imaginable project.
But even if you start with simple aims, one of the near-universal principles of research is that it tends to get more complicated over time. Every source you read creates more reading, every bit of data you gather creates more analysis, every analysis raises new questions, and it's very easy to end up overwhelmed by the sheer volume of work that your previous work has created.
If deadlines are approaching and the pressure is starting to build, you may be forced to re-evaluate your goals and simplify your project. There are two ways to approach this. The first is to take your existing plan and try to trim it down, but this can be quite hard to do since everything you cut feels like a loss. Also, since the starting point is something you haven't yet achieved, there's no guarantee you'll be able to reach this lower goal from where you are now.
The second way to simplify is to start with a blank page and add things. Given what you already have (skills, resources, existing results), what can you add to move yourself closer to an achievable, worthwhile end? This is a much more positive way of thinking and can help you focus on realistic short-term goals, which you can then build upon.