"I find it hard to believe..." actually means that you don't believe something. The first part, "I find it ___", means "I think it is ___":

I find it hard to relate to him sometimes.

I find it really hard to focus on my work in the afternoon.

Use "I find it hard to believe..." to complain about something that you don't think is true.

Grammatically, you should follow "I find it hard to believe" with a clause connected by "that". The clause is like a little sentence:

I find it hard to believe that you wrote this paper yourself without any help whatsoever.