Without Prejudice is often not properly understood and is very over-used especially by non-lawyers.
In general, it is not in your interests to say that your communications are Without Prejudice. Most litigants-in-person are trying to make straightforward reasonable settlements. There is no reason to hide the fact that you have attempted to be reasonable from the court.
Most often, professional lawyers, when dealing with litigants-in-person, are trying to shaft you. They would not want the court to know what they are up to in any event so without prejudice is useful to them and they use it a lot.
Are you ashamed or embarrassed by the contents of your letter to the other side? If not, then don't use Without Prejudice.
What you should do is to make sure that offers are made without admission of liability. This is far more useful to you.
Very often, the use of Without Prejudice is a pompous affectation and has no relevance and could turn out to be a disadvantage if you want to show a court how reasonable you have tried to be in your dealngs