Very much the same story for me. This year was my first proper go at structured training and I felt like my FTP improved a decent amount but ultimately, for long days in the saddle, I just could not stave off the inevitable bonk. I suspect that a good deal of the blame was due to a lack of long slow miles, and therefore a vast lack of efficiency (as well as other limitations).
To experiment, 2020 is going to be a year of base miles for me, sprinkled with some decent, high yield intensity.