Learn English – “Get out of your own head”

Back in the days when ovens were often powered by gas with no added chemicals to make it smell bad, a purported method of suicide was to stick your head in the oven and turn on the gas without lighting it (the modern equivalent would be running your car in a closed garage). While that was quite a while ago now, I can't think of any other meaning this phrase could have.

Exactly what this translates to in terms of how well John is doing depends on his personality. If John is the sort to joke about committing suicide, I would probably take the meaning as: things aren't as bad as they could be, but they're not very good, either. If John seems like the type of person who wouldn't joke about committing suicide, you might want to take this much more seriously.

According to the article Instruments of Expression: Bells, Drums, and a Horn, it refers to the practice of heraldry. It comes from the sense of 'horn' as a trumpet, and one who blows his own horn is someone making great fanfare about himself, as is usually more appropriately left to a herald.

It says specifically:

To blow your own horn is to be a braggart or "blowhard." This expression, arising in the American West about the middle of the 19th century, derives from an earlier expression, blow your own trumpet, dating back to at least 1576 and probably originating in medieval times, when heralds blew trumpets to announce the arrival of the king. Of course, any merchant or other commoner who wanted to announce his arrival had to blow his own horn.