Set yourself clear milestones. When will this project be "done"? What features is the engine going to have? What is in/out of scope?

And be aware, as an indie you either make a game engine or you make a game. It's near impossible to achieve both without sinking a crazy amount of time into it.

Making a game engine is an excellent way to sharpen your skillset when it comes to programming, math, physics, and graphics APIs. But it's an awful way to make a game. Just be sure you're going into it for the right reasons.