He quit on our marriage

I just realized something today.  I've been thinking about everything thats gone on, what he's said and listening to a lot of different advice.  I realize now that my husband actually quit on our marriage 10 years ago.  He stopped caring about me, stopped showing me love and respect, starting doing his own thing and creating a life for himself.  I think for the past 10 years he's been telling himself that all he has to do is wait for our youngest to be 18 then he can leave.  And that's exactly what he did.  One month almost to the day after our son turned 18 he told me he didn't love me anymore.  I think he thought if he just stuck around to be there for his kids, then when it came time to leave he would be the "hero" for sticking it out for his kids.  Only it backfired because they hate him for what he's done.  On my birthday 9 years ago he didn't even acknowledge the day - not with a card or a gift or even a word.  I remember this because we were on our way back from a 4 day trip up north.  That same year he didn't acknowlege Mothers Day.  I realize now that it's because I didn't matter to him, even back then.  So I ask myself (and I asked him) what was the last 10 years then - a lie?  Was he just using me for the sex and the mother figure?  In all those years I beg and pleaded with him to try and save our marriage.  I wrote letters, I wrote poems, I sat him down with one on one talks and not once did he tell me what he was thinking.  Even 3 years ago I said to him, "it feels like all you're doing is waiting for the kids to get old enough before you leave" and he replied "I have no plans to leave".  But he did.  He did have plans to leave when our son turned 18.  All these years he convinced himself that 18 was the magic number and when it came he knew it was time to go.  I will never understand the reason why.  I will never understand why he didn't talk to me, why he didn't give us a chance - why he quit on us.