It is not clear from your question why you think you will most likely lose your equity share of the house. How was the house acquired? Was it acquired with community funds? Your separate funds? His separate funds? These questions must be addressed in order to provide an answer.
I would need more information to provide you with an informed answer. Merely moving out of your house should not forfeit any equity in the house unless it is based upon some fault ground. If the house was purchased with marital/community funds during the marriage, then you will not lose your equity share of the house.
I'm not sure what a notarized letter could/would do for you. It would depend upon more information.
Moving out has no impact on your legal right to an equitable division of the home. To secure your rights, you should obtain a written agreement from your husband. A notarized letter is of no value since a letter, by definition is a statement by one party. You need an agreement, which is a document both parties agree to.