The answer is that being a full-stack developer is not hugely difficult, but you require quite a wide range of skills. But you can build up to become a full-stack developer. You can begin with the front-end and work your way back to the back-end; work with databases; get your hands on a fully configured website, and create a user interface for a smartphone app! Each of these builds up on the experience of the other, so if you plan this over a sufficient period of time, let's say a few months for each, you should get good experience within a couple of years.