The problem with the population in Bulgaria is that the people of working age are rushing to emigrate to richer European countries, leaving the elderly behind, who of course are not contributing to the economy. This makes the country poorer and therefore encourages more and more young people to emigrate for a better life.
Then the married folk take their children to settle in a richer European neighbouring country, so they grow up expecting a life style which these richer countries can offer. Why, then, would they want to go back to Bulgaria to live? There is little to attract the except their origins there.
With an economy declining and the age average increasing, there is little hope that Bulgria will profit and change upwards. The young people emigrated or wanting to emigrate as soon as possible, the ones left being the least adventurous or fit or young, then the population decreases. It's a bad situation.