just so you understand what the basic rules are; as an individual you cannot be forced to provide testimony against yourself (edit:) in a criminal case, but you can in a civil proceeding.
But your written records are fair game as evidence, including your personal diary.* The right not to testify is really more about "so there's no incentive to torture you" more than it is a civilized notion that "we are gentlemen, let's have a level playing field, my secrets against yours". Then in addition to that, there are also document retention laws for documents that seem like they will be pertinent in the future, court orders during litigation that are even more restrictive, and corporations held to higher standards (there isn't generally going to be evidence of a person lying to himself, but corporations inducing employees to lie is more directly in the crosshairs)
*from a quick search "Even if a diary doesn't contain relevant evidence, courts will usually allow the side requesting to see it to make that determination for themselves. Particularly in cases where a party is alleging physical or emotional injuries, diaries are rather helpful in understanding what a person has experienced due to their injury. If a party kept a diary before the injury occurred, it can also help to show the contrast between life prior to and after the injury."
Hoping you’re not a lawyer, or that you just in-artfully phrased that… because a person cannot be compelled to provide testimony against themself in (or relating to) an unadjudicated criminal matter pursuant to the 5th Amendment, but a person can absolutely be compelled to provide testimony against their interests in a civil matter so long as there are no possible criminal charges from whatever the testimony is. They can also be compelled to testify about a criminal offense that has been adjudicated. Case law has made it clear that the 5th Amendment is a narrow exception to the obligation to provide testimony to a court in civil matters. Also, invoking the 5th Amendment in a civil matter can, at times, be used against you, unlike in a criminal matter.
* this not legal advice and there lots of nuances to the 5th Amendment right to not testify.
But your written records are fair game as evidence, including your personal diary.* The right not to testify is really more about "so there's no incentive to torture you" more than it is a civilized notion that "we are gentlemen, let's have a level playing field, my secrets against yours". Then in addition to that, there are also document retention laws for documents that seem like they will be pertinent in the future, court orders during litigation that are even more restrictive, and corporations held to higher standards (there isn't generally going to be evidence of a person lying to himself, but corporations inducing employees to lie is more directly in the crosshairs)
*from a quick search "Even if a diary doesn't contain relevant evidence, courts will usually allow the side requesting to see it to make that determination for themselves. Particularly in cases where a party is alleging physical or emotional injuries, diaries are rather helpful in understanding what a person has experienced due to their injury. If a party kept a diary before the injury occurred, it can also help to show the contrast between life prior to and after the injury."