Why do I always feel like I should be doing something else?

That is a question you can only answer yourself. When you wondering why you should be doing something else, you are wondering about other ambitions you have that you want you fulfill, or needs that you have that are not being met. The only way to answer your question is for you to look inwards and wonder about what it is that you really need. Allow yourself to place yourself outside of others' expectations and wonder what is you really would be doing without those. Self-reflection is instrumental to really understanding to who you really are, and what it is that you want.