At work, you always get sleepy after lunch and don't work very efficiently. You are describing this problem to a friend of yours:
I find it really hard to focus on my work in the afternoon.
This phrase is used to describe opinions or feelings that you have. It's a slightly educated and formal-sounding phrase that you use to emphasize that what you're describing is your personal experience, not a suggestion that you are making to the listener. Here is another example:
I find it interesting to sit outside and watch the different people walking by at lunch time.
The word "hard" is especially common with "I find it":
I find it hard to believe that you're really going to need all that luggage just for 3 days.
To "focus on" something means to pay attention to it. When you're focusing on something, you only think about that topic and avoid thinking about other things.
"Focus" can also be used by itself, without "on ___":
I find it hard to focus in the afternoon.
In the sentence above, "my work" doesn't mean your job. It means the specific tasks that you are supposed to do at work.
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