The main thing which is not fulfilling in my full-time job is the fact that I'm selling my time which makes difficult for me to spend my time for myself whenever I want.
Also, I learned that if you are so unhappy with your current life, mostly you will not be happy even if you get what you want. I changed my daily routine to include lot of physical activity and reading, which has greatly improved my mindset and is helping to be more active in my day-to-day life.
As an aside, like you I had a very early (around 21,22) quarter-life crisis after my college (Now I'm 28). After working in a unfulfilling job, I just quit without any plans. Looking back, it was the wonderful time of my life and it reinforced my want to create my own business, so that I don't sell time rather sell something based on my expertise. It was scary enough; case in point, I'm currently in a full time job. But soon, I may take a leap to have my own business and an independent life.
All the best.