A fascinating set of questions. Let me start by distinguishing atleast two: 1. the issue of 'entwined' lives and their relation toindividual autonomy. 2. The implications of this for 'ownership' ofautobiographies.
The first of these is only a problem if we start with theassumption that everything that happens (in the human world) mustbelong to one and only one agent. As the saying goes, 'it takes twoto tango'. You wouldn't have been 'free' to write about arelationship if there hadn't been another person! You were, in asense, co-authors and co-owners of the events of the relationship.
The second question is more difficult. In fact, I think theexample of the joint blog is not really appropriate. A blog is in thepublic domain, and is thus not a 'secret'. Your blog has been takendown but then the real moral issue is about respecting the wishes ofsomeone who has changed their mind, and not about my 'ownership' ofmy own life. A better example would be intimate secrets that werenever made public, and where the question of making them public neverpreviously arose. Only then does the problem of my freedom to do whatI like with the events of my own life arise in a pure way. However,the answer to the first question suggests that there is no paradox insaying that the events of your life belong to you and to otherpeople.
So, I think you are right to feel moral qualms. Events of therelationship (such as writing a diary) are co-owned, even if you didthe writing, and thus you have a responsibility to your ex-partner.Of course, the purpose behind disclosure might matter (making publicthe diary as part of a legal proceedings of some gravity, forexample, might be morally compelling). Certainly, I would askpermission.