Your last semester of undergraduate work probably consisted of writing little policy briefs using your economics skills. Your professors probably told you how wonderful you were. Everything was easy and fun. You had a good time. Graduate school is completely unlike that.
Graduate school is much more analytical.
You are already expected to think like an economist. Here you learn the analytics of economics. It is much more quantitative than undergraduate work. There are many more mathematical proofs. Everything you say and write will be dissected.