Thanks everyone for the nice comments.
(About my post at 12.54 on 1st April. Page 12 if you have 25 posts per page!)
And I do know that it's not like this for everyone, (staying with someone you are not in love with, or have fallen out of love with,) and some people do have a genuine love for each other. But I don't think it's very common.
As some posters have said, many people fall for each other due to lust, and the excitement of a new relationship, and some probably do love each other, but for some it does turn to compromise, sacrifice, boredom, and indifference (as the OP said.)
Someone said earlier that it's not too late for me to leave my DH, but tbh, my marriage (and my life) is not that bad. Even though it was a struggle in the first 10-12 years of our marriage (and he was a bit of a shit then,) the relationship is better now (25 years or so on...) It isn't awful, or abusive, and as I said, we do have some decent/good times, even though he is a bit moany !!!
As I said earlier, things could be worse, and some people have much worse lives. But it does make me a bit sad that I have never felt deep genuine love for any man. I feel a special kind of deep love for my daughter though. So at least I have that. 
Also have a lot of admiration for people confident enough, and secure enough to remain single, and not feel forced/coerced into being in a couple, just because society tells them to.