Filtered through my own personal perspective, being someone in a poly relationship who is not completely "out", it seems to me that there are two separate issues...
It ended because I was not brave enough to embrace the lifestyle, therefore it was not possible for me to exist happily living it just partially. Keeping her a secret always felt so wrong, ...
By not considering myself brave enough to be honest with everyone in my life about our lifestyle does that mean it does not mean enough?
I am reading that you feel that you were not "really" doing poly if you feel the need to keep it a secret. I, personally, consider myself "fully" poly even though we are not "out" to the world at large - and won't be, at least until I retire, due to my profession (morality clauses and whatnot).
I can't say that this particularly bothers me - there are many areas of my life that I keep "private" from the world at large. But then again, I am a generally private person - I do not develop social friendships with coworkers, for instance. I tend to divide people into three spheres - professional/public, family/acquaintances, chosen family/close friends.
How I present/interact with Dude (or my female FWB) depends on which "sphere" I am in. In a professional/public context - he is my husband's best friend who I am also close to. I might tell a story about a meal he cooked for us, we might be seen eating or shopping together (with no PDA), I will introduce him as "our friend" if I run into people while we are out.
In a family/acquaintance context - people know that he lives with us but not that we are "together", I might tell a story about how he answered the door in his underwear, I will refer to him as "our roommate".
In a chosen family/close friends context - people know he is my "boyfriend", I might tell a story about a funny thing that happened during sex, or talk about how our feelings have evolved over time.
These levels feel natural to me. I don't think "everyone in my life" - from my boss to the maillady - has a right to know my personal business. My family is great - but they are related to me by accident of birth, not by choice. Acquaintances may be nice people - I interact with them around certain activities or talk with them on limited topics. Just because family/acquaintances share some aspects of my life, doesn't mean that they have to share ALL of them. Chosen family and close friends are the only people who, I think, are entitled to the "real me" - otherwise they aren't chosen family/close friends - these people love ME. They might not agree with me but they get the whole ME.
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This, I think, is a separate issue...
having such limited time for intimacy and true closeness was hard for her and in the end not fair.
I must have missed WHY there was such "limited time" ... and what this has to do with the privacy/secrecy issue above. When I need one-on-one time with one of my boys I ask for it. I invite them to take a walk in the woods with me, or retire to the bedroom for some cuddle-talk, or ask to take a long drive. Admittedly this is easier to do on the spur-of-the-moment because we live together but...
I can understand that having kids would further complicate the picture but I don't see this as an insurmountable problem (for the record - we don't have any). Just because you are a parent doesn't mean that you aren't entitled to a private life. There are plenty of (non-poly-related) activities that are for "adults only" - it's okay to set boundaries.
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Reading your posts it seems like the problems stemmed from communication issues amongst the three of you regarding getting each person's needs met within the context of the relationship(s). The secrecy/privacy issue seems to be a complicating factor that you focused on rather than the underlying issues. Just one person's perspective.
JaneQ