Why are so many marriages of 20 or more years ending?

my marriage of 25 years ended in 1999. i dealt with it initially but there was only minimal contact with the kids whom one of i live with. recently they were made aware that their father remarried. someone younger with two girls same as themselves only 12 and 16. this hurt them especially the youngest and me. it was like opening a semi healed wound. after 7 yrs he refuses to have contact with me or even care about me.
he said he would never marry again and he still blames me for the relationship ending yet i didn't even see it coming and am still at a loss. there are days i think the life i'm living now is just a nightmare and the one i remember before all of the hate and ugliness started i will wake up in again.
people think i'm nuts for not moving on. i did for awhile until the new marriage started a whole lot of new crap with contact and christmas, plans, etc. i miss the family i once had. i don't trust many now. i'm afraid--he made me that way before he finally moved out.

Marriages end because people become selfish and do not care about working as a team with their spouse. They also can be shallow and fall out of love with their spouse's physical appearance because they are full of their own desires and don't care about the other person's feelings at all.
A guy like this is not worth your time. Find some things that you enjoy doing that make you happy and fill your heart with joy, so that you can get your mind off of him. He's not worth your pain if he did this to you.

Many marriages of 20 years or more are ending because, as women age, their appearence gets uglier and uglier, and their personalities become nastier and nastier, so that eventually, the need of the husband for a wife for sexual purposes is overcome by his revulsion to her aged appearence and her nasty personality. He then either leaves her or divorces her.