I am so in my head?

Do you guys like Carly Rea Jepsen?

I chanced to hear(listen to?) a song 'I really like you' by the singer,
and the song sounds pretty good to me.

https://www.youtube.com/watch?v=92hNq0NUPgc

I glanced at the lyrics, and one line was quite hard to understand.
Let me show you what that is.

Late night, watching television
But how'd get in this position,
It's way too soon, I know this isn't love.
But I need to tell you something,
I really really really like you.
I want you. Do you want me too?
Oh, did I say too much?
I am so in my head when we are out of touch.

The term 'position' probably indicates that those two little birds made love, I guess. Well, that's not the point.

What caught my eyes was the part 'I am so in my head',
as this was completely new and puzzling to me.

I entered 'so in my head' to see whether this is a fixed and common phrase in vain. It didn't seem so.

I turned to one of my buddies good at English,
and his reply was that it's almost same as 'I fucked up',
which I could hardly believe to be right.

Would you please help me figure out what that is
and under what circumstances that expression is used?