Generally, one good way to lose a case is to keep your lawyer in the dark about damaging facts. If you have done something untoward or embarrassing, and you don’t tell your lawyer about it, this means that your lawyer will not have an opportunity to prepare to deal with it at hearings on your case. When your lawyer is surprised at the trial, it likely is too late to construct an argument to counter the embarrassing situation, and probably too late to try to bring in witnesses to present the matter in your best light.
If the other side knows something about your case that your lawyer doesn’t, the lawyer will be unprepared to defend you on that matter. At least, if you tell the lawyer about the problem well in advance of hearings, he or she can help you decide what to do to try to avoid a bad outcome, if possible.
What you tell your lawyer is confidential and is not to be disclosed without your permission. You should never fib to your lawyer or withhold information from her or him! Ordinarily, these kinds of things will come back to hurt you. So, please make sure your lawyer has all the relevant facts in your matter, not just the ones that make you look best, but all of them.