I have peace to weigh your worth, now all is over,I'm not, in general, a big fan of Rupert Brooke. I find him too old-fashioned, too archaic. This one sonnet, however, remains a personal favourite - not so much for the 'poetry' (the verse is clever but hardly moving) as for the fact that it captures a fundamental problem with moving on, the logical flaw at the heart of nostalgia. In order to have fond memories of something you no longer have / are - you must enjoy both the presence of something and its absence - a difficult argument to make unless you take a heraclitean view of the self and argue that the self that enjoyed something in the past is different from the self that is happy to have left it behind in the present. This is not a problem only with relationships, incidentally - it works equally well for childhood keepsakes, former employers, schools, etc.
But if to praise or blame you, cannot say.
For, who decries the loved, decries the lover;
Yet what man lauds the thing he's thrown away?
Be you, in truth, this dull, slight, cloudy naught,
The more fool I, so great a fool to adore;
But if you're that high goddess once I thought,
The more your godhead is, I lose the more.
Dear fool, pity the fool who thought you clever!
Dear wisdom, do not mock the fool that missed you!
Most fair, -- the blind has lost your face for ever!
Most foul, -- how could I see you while I kissed you?
So...the poor love of fools and blind I've proved you,
For, foul or lovely, 'twas a fool that loved you.
Rupert Brooke, 1913
Though even here Brooke gets it a little wrong, I think. The problem is not so much that in condemning things from our past we condemn ourselves (most of us are comfortable saying that we behaved like idiots once upon a time - the past is always a convenient scapegoat); the trouble is that we may not always want to condemn things that we have left behind. Just because I personally have moved on to something else, does not mean that I don't value the person I was / the things I had - my old job may not have been what I wanted to spend my whole life doing, but I thoroughly enjoyed it while I was there and have no real regrets about the time I spent there.
This may sound fairly reasonable, but as a point of view it's pretty much a tight-rope. Say you're talking about an ex-girlfriend. Talk about her too fondly / praise her too much and people will instantly assume that you still have feelings for her, that you would like to get back together with her and that you're fairly unhappy with the way things are. They'll begin to feel sorry for you. They'll assume that it must be she who dumped you because you're clearly still in love with her. They'll monitor your speech closely for signs of lines from Devdas.
At this point the pendulum will swing. You'll start by insisting that you have no regrets and that you're quite happy to be out of the relationship. This will be greeted with that pitying, tearful look that says "It's okay, you don't have to deny your feelings to me, I understand how you really feel". Probably the most irritating look in the world. You'll lose it. You'll start detailing all the things you hated about her. You'll go on and on about how irritating she was, how miserable you were, etc. Before you know it, people will be asking you why you're so bitter about it. You'll get advice about how you need to get over it, achieve closure, forgive and move on. They'll give you the horrified look that says - you were in love with this person, how can you talk about her this way now? You'll think about it and be a little horrified yourself. You'll start talking about the her good qualities. You'll praise her. And the whole cycle will begin again.
This cycle is even worse if you happen to be talking to your ex's new boyfriend. Now it's not just about you - it's about him as well. If you praise your ex too much the new BF will be suspicious, he'll be convinced that you resent his presence in your ex's life. He'll watch lynx-eyed for you to try and usurp him. Even a genuine "You're so lucky!" will be met with indignation - are you trying to say I don't deserve it? That I'm not good enough? Huh? Huh? Like you were.
Go the other way and talk about how your ex is so lucky to have found someone so great and the new boyfriend will get all defensive of your ex. Excuse me, this is my girlfriend you're talking about - just because you were a jerk and didn't get along with her is no reason to badmouth her now, etc. This is the point where you throw up your hands and decide you can't win.
Which leaves us with the most fundamental question of all. At the risk of miffing people who worship When Harry Met Sally (yes, there are actually people like that) the real question is not whether a man and a woman can ever be just friends (of course they can) but whether they can be friends after they've been in a relationship. That's the one I'm still trying to figure out.