Even if the character had more than one mother, it’s safe to assume the diary belonged to only one person. Add an apostrophe before the s in mothers for the singular possessive form.
Change the comma after diary to a period.
There is never a time when the first person singular pronoun I should be lowercase. Also, the contraction for I am needs an apostrophe.
Get rid of the ellipsis; there’s no reason for that sentence to trail off.
I found my dead mother’s diary. I wasn’t going to read it, but I’m so glad I did.
If it were up to me, I’d change the second sentence to make it flow more smoothly: I wasn’t going to read it at first, but I’m so glad I did.