The following sample letter to give one's spouse was composed by two longtime family therapists and a divorce lawyer, all of whom have been deeply involved in changes to make their professions, community and society more supportive of marriages. More background information here.
I love you, and I have grave worries about our future. I don't feel I have done a good enough job explaining myself to you when we talk. So when I came across the idea of sending you written notice of my concerns, I decided to send you this letter in hopes that we can save our marriage before it is too late.
I am giving you this written notice that our marriage has serious problems that may lead to separation or divorce. I do not want our marriage to end and will work with all my might to preserve it. But I need you to recognize that our marriage is on the brink of divorce and to work with me to save it.
I'm still committed to this marriage, and would like to build a strong foundation for a lifetime of marriage with you. But ignoring issues and problems will probably make them worse. I want to do this hard work with you now, before it's too late.