I think I almost had a lucid dream?

I had a dream at first that I was looking at myself in the mirror but it was like a younger version of myself, like maybe when I was in my teens (I'm 22 now) and then I turned around and saw a door open to a hallway that had green walls, and I remember thinking that this was real. But before I had a chance to go out into the hallway I was too afraid because I think I realized the hallway lead to a dream in which I could control whatever was I going on.

I wonder if you can transition from a regular dream to a lucid dream or if most people just go right into a lucid dream.

Usually, it's like a snap realising you're dreaming, either by reality check, awareness or something else, and in my experience everything just gets into focus. You were almost there, though! I think with the sudden scare, maybe the dream was trying to trick you by messing with your emotions.

That aside, you're really close to a lucid!