What a wonderful mama! I loved reading your post because it demonstrated you are attuned to your kids and care deeply about healthy attachments:) This tells me you will figure it out soon in your own beautiful way!
But you asked for specifics, so I'll throw out some ideas to think about alojng with other posts. Maybe you can find 15-20 minutes a day to spend alone with your oldest and talk about all of the wonderful traits about him you would like his help with teaching his younger siblings? You could do this by talking with him about his daily experiences and/or doing some sort of activity he likes to do and let him know the ways in which you see and hear him. Not necessarily overly praising him (want him to experience the genuine nature of legitimacy of things you say you appreciate and why), but said with smiles and appreciation for his specific contributions to the family and you as his mama.
Maybe you can find a way to have both of your boys experience specific roles they can both feel pride in with their new sibling. It will be an adjustment for both.
One thing I guess I'm curious about is why the sudden shift in self confidence of your oldest? You are probably right in what you think is going on but are there any other possible experiences that could have influenced this? Maybe look into the bigger picture stuff too...school, friends, acts same around different people or vice versa. Just to make sure nothing else outside of your immediate family is negatively impacting him.
The tendency with really really good people like yourself is to view yourself as the possible cause of our children's struggles. While this is so important, to be self aware, be careful not to overly self assign blame. Your gift is you have awareness so will never have to worry about being ready and able to meet your children's changing needs. But the focus, if on you too much might be a distraction to things unrelated to you that you can help your children rebound from (peer relationships, school, etc.).
If I would've had a mother like you sound to be...I would have been very lucky indeed:) Congratulation on your children and gosh, hang in there you are almost there!!!
PS
Have you discussed this with your partner and close family to see if they can also take time out for each of your sons? Sometimes you can only do so much, especially adding a newborn to the mix...this might also ease your worries about your own limitations so you can find your own balance. Though you sound like a super mama, you are human:)