It is much easier to use OpenXML classes then writing xml string. Try this:
using (WordprocessingDocument document = WordprocessingDocument.Create(fileName, WordprocessingDocumentType.Document))
{
MainDocumentPart mainPart = document.AddMainDocumentPart();
mainPart.Document = new Document(new Body());
//... add your p, t, etc using mainPart.Document.Body.AppendChild(...);
//seems like there is no need in this call as it is a creation process
//document.MainDocumentPart.Document.Save();
}