(一).I'm under a lot off pressure.我的压力很大a /我觉得负担很重。
This expresses that "I'm feeling very stressed." Usually people have this feeling when they have a lot of extra work to do.
put (a lot of) pressure on 给......施加（很多）压力
I'm under the gun.我备受压力的煎熬。
原音重现：

A: I'm under a lot of pressure at work.
我工作的压力很大。
B: Oh? Do you have to work on an important project?

哦？是吗？你在做一个重要的项目？

(二).It Couldn't have been easy. (对你而言）不是容易的事儿。
This expresses that "It must've been hard for you." The speaker thinks a person had a hard time with something.
It hasn't been easy.不容易。
Things haven't been easy.(虽然我克服了 困难并取得了最终的胜利，但）很不容易。
原音重现：
A: How did John become so rich?
约翰是怎么变成富翁的？
B: I don't know. It couldn't have been easy though.
不知道，肯定很不容易。