"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 ___":
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.