Tai Ji Chuan has a long history and a wide range of knowledge, which makes self-study more difficult.
You can't be sure whether they are correct or not, let alone practice internal qi, unless someone who knows the ropes points out your actions to you.
Second, Tai Ji Chuan emphasized that it is futile to have a boxing routine without the guidance of a wise man.
If you have enough time and firm will, it is not impossible for you to study by yourself, but it takes a lot of time and energy.