400 miles a month makes for a good base. That is if you do it for minimum 3 months and even better six months. Try doing 200 miles, three hundred miles, four hundred miles. then do 250, 350,450. Do a stair step and cycle back and start the progression over. Don't be too bummed about getting dropped on the first climb. It will take time. Riding hills all the time will not get you there. You need a mix of hard and easy days with the weight being on long easy days to build cardiac endurance. Of course it is more complicated than that. I would suggest being very recovered before doing anything intense. You will gain more from it if you are fresh.
Don't be embarrassed. Your failure on the hills was not a character defect. No one is judging you.