В предложении "I used to think it would be .........such a career, but I found out what I needed to do and I realised that it's within my grasp" пропущено слово, которое описывает, каким казался карьерный путь ранее. Судя по контексту предыдущего абзаца "I used to think it would be impossible to pursue such a career", пропущенное слово – impossible.
Таким образом, предложение будет выглядеть так: "I used to think it would be impossible such a career, but I found out what I needed to do and I realised that it's within my grasp."
Ответ: impossible