I feel crushed....

Short-Time player here. I was inspired to start CK2 after reading the great stories of peoples experiences on this subreddit, it took me a while to get to grips but i feel i have a good concept of all the game rules and know what to do.

I started, like many, as Earl of Dublin and worked my way up to be King of Ireland, Scotland, and Wales.

It took me around a week of playing 3-4 hours a day, I had 4 main vassals, all duke level holding all the power, i kept them all extremely happy by giving them all the counties/duke titles. All three kingdoms were an Elective so because they loved me they voted my son to be heir repeatedly. It was a brilliant cycle, until one day they suddenly changed tack and decided to hate on me, vote themselves as heirs, my son inherited only wales and the 3 counties in my desmene... I feel crushed.

So any tips for a noob? What did i do badly?

Also advice on marriage/claims would be good, i tend to just marry someone with good stats, I dont marry for claims.

My main method of war is having De-Jure claims and Fabricating claims.

Thanks!