I have read much of the Diary. My wife read the whole thing and is a great proponent of reading it.
However ----- The effect it initially had on me (after reading perhaps 10% of the book) was one of extreme discouragement. St Faustina was SO holy, almost beyond comprehension. Page after page of revelations, personal sacrifices, etc. How could anybody be that holy? I’m past that point of discouragement now, but I would not recommend a scrupulous person to read the whole book. Selected excerpts - maybe.
But I’ll pray for you that if you read it, that it will be good for you!