I am not at all a 'self help lit' fan, this book may sound to be in that genre but it is actually written by two established Jungian analysts and I've found it hugely helpful to me.
'Living your unlived life' by Robert Johnson and Jerry Ruhl, 2007, Penguin.
The sub-title is 'Coping with unrealised dreams and fulfilling your purpose in the send half of life' 
I like the way in which they frame such angst as many seem to be suffering on this thread, not as something to suck up and womanfully soldier on through, but as a useful poke in the ribs, something to be welcomed, to wake us up, call us to attention. Apropos BoffinMum's example of her colleague's sad resignation of his racing driver career ambitions, rather than dismiss such a dream as ludicrous from the start, as one might do, I would ask that chap to break down what it is about being a racing car driver that so fired him up. That's what Johnson and Ruhl get you to examine. Not to relinquish dreams per se but to get to the very nuts and bolts of what it is about that fantasy that so speaks to you. Is it adventure, risk, speed, others' admiration and attention, what is it that in its essentials is switching you on? I find StickWithIt's post very wise, she has boiled down what really gives her pleasure and actually, it's often very very achievable.
Another example, from Johnson and Ruhl, the classic mid-life urge to have an affair, is in many cases more often about fear of ageing, fear of loss of sexuality, of becoming invisible etc etc and there are ways to answer those deep anxieties and needs without jumping the bones of the first (probably highly inappropriate) person one encounters. For some those deep needs might be better answered by the focussed overhaul of personal image, dressing better and walking tall, feeling visible and desirable again after a long stint in dowdy, self-deprived motherdom. I have a friend who, in her early 50s, has for the first time taken up amateur ballroom dancing. She's having a blast, loves the performance aspect of glam frocks and loads of slap, the camaraderie, for her it works, it fulfils deep needs, makes her feel vibrant and powerful and more 'herself'. It's not about telling one's self to get a grip and batten down one's 'unrealistic' fantasies, it's about being kind to one's self, listening, really listening, and then thinking hard and creatively about how to go about fulfilling that need in a way that doesn't rip yourself or those who depend on you apart.