Thank you for all of the replies. A mix bag and all very interesting to read!
Whatdirection You are not derailing at all and I am so sorry, that is a hellish situation to go through. Unforgivable that he behaved so deceitfully towards you after such a long time together. And particularly hard when he hasn't acknowledged the enormity of his betrayal . Wishing you strength and (ultimately) peace. 

Angrymum22 we felt the same way about our wedding and like Country girl and her DH, I think we share pretty much the same values.
I don't think most of us are just marking time in shared misery for the record I don't automatically think that either Lemonroses but I suppose I have come to a realisation that a long marriage is not necessarily all tea and cakes either!
Some very good points about compromises and, most importantly, whether they feel worth it. And of course this must be seen in the context of what one can bring to a relationship as well as what one takes from it.
The comments about losing yourself in long marriage and not being able to see the relationship clearly after such a long time certainly resonate with me. I think that is why I started this thread really. Our eldest is about to go to university and it is a time to reassess I suppose.
And of course it is lovely to read the posts about the happy, long marriages, from Lynsey91 and other posters! You never hear much about them on the Relationships board, or on Mumsnet in general, so it is good to know they do exist, and that there are more out there in the ether than one might imagine! 
In fact there are so many posts about how awful some dps and dhs are on this site , that it makes me appreciate my DH more., not less. At the same time, of course Mumsnet is useful in that it is one of the few places where women's rights (and I use that phrase unapologetically) are discussed and taken seriously and I have certainly learned a lot from reading about other women's experiences here.
Thanks all!