Not only is truth a defense, but also your opinions, perspectives and analyses are protected by the First Amendment. Libel cases can properly be brought only if you negligently (or in the case of matters of public interest or public figures, recklessly) make untrue factual statements. Although many libel cases are initiated, plaintiffs are successful in only a small percentage of these cases.
When libel cases are initiated, the plaintiff usually sues both the author and the publisher. Thus, publishers have extensive experience in protecting against libel suits. Sometimes, publishers make suggestions about changes to a book that are made for the purpose of avoiding a libel suit. Many authors resent these suggestions, and at times the discussions between a publisher and and author about suggested publisher revisions can be unpleasant. Although this process may be viewed by the author as unpleasant and interfering with artistic or academic freedom, it is a process which may save both the author and the publisher substantial amounts in legal fees that might otherwise need to be expended in a law suit.
Finally, I must say that politics play a major role in these situations. The lines between assertions of fact and assertions of opinion can be hard to decipher. Unfortunately, an increasingly conservative judiciary has had a tendency to provide less deference to artistic freedom than was true two decades ago, which has improved the chances of success for plaintiffs in libel cases. Nonetheless, we have a strong liberal tradition in this country in support of the values embedded in the First Amendment, including freedom of speech, freedom of association and freedom of religion. This liberal tradition in support of free expression usually results in defeat for a plaintiff in a libel case.
Interestingly, Congress recently recognized the importance of limiting the scope of libel claims when it enacted by overwhelming bipartisan consensus the Libel Tourism Act, in which U.S. courts need not enforce libel judgments obtained in foreign countries against U.S. citizens or U.S. based companies where the standard for proving libel in such foreign countries does not comport with our requirement that there be a showing of either negligence or, in the case of public figures and/or matters of public interest, recklessness. This legislation reflects the fact that in the United States, courts are expected to reject libel suits except in fairly egregious circumstances. This is certainly good news for anyone who is preparing to publish a memoir.
Having said all of this, I must state that the advice that you obtain from this site is no substitute from retaining legal counsel to review the specific facts and circumstances of your case and provide specific advice to you. The investment that you make in retaining legal counsel will pay dividends in the long run. You certainly should not use "self-help" based on internet web-site advice to guide you through the often complex legal ramifications of situations such as this.
Nothing immunizes anyone from lawsuits, although you can minimize the chances of a lawsuit being successful. A disclaimer is a necessary but not necessarily sufficient factor, as is changing the names.
Changing names won't help if the people are recognizable, as it's still possible to defame people whose names are changed.
Similarly, sticking to what you think is true may not matter if others disagree about what's true. Also, truth is only a good DEFENSE to a libel (written defamation) claim, but that doesn't mean you wounldn't get sued and have to use that defense.
Your state also probably has privacy laws including disclosure of emabarassing facts, which can include embarassing truths, and and "false light," showing someone in a techincally true but purposefully unflattering way.
If you've got a publisher, they'll insist that you warrant and represent that your book doesn't infringe on anyone's rights, and require you to agree to indemnify them if that's not the case. If you're self-publishing this memoir, you need to hrie a lawyer to vet it, to accomplish the same purpose.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Truth, of course, is a defense to defamation claims. And you have the right to publish your memoirs if you choose to do so and find a house to publish it. The general disclaimer you have won't protect you from lawsuit -- nothing can do that, unfortunately -- particularly since you are presenting the work as non-fiction. Perhaps you are thinking of the disclaimers at the beginning of novels, that start off along the lines of "This is a work of fiction. Any resemblance to real people, living or dead, is an unintentional coincidence, as none of the people in this book are real etcetera." Traditionally, authors who want to tell dirty secrets (even true ones) write a roman-a-clef, which is a work of fiction including characters who are thinly disguised real people. While this doesn't immunize the author from lawsuits either, it provides at least one more avenue of defense ("I said it was fiction . . . that character is NOT you.")