Oh my good lord, you're anything but. It is often when children need us the most, and are at their most demanding, that we find ourselves the least able to cope and the most desperate. This is TOTALLY normal and you are from the sound of it a really excellent parent.
If you didn't care you wouldn't even be on here. 
I do understand, and also I do think it'll get easier for you. Fwiw I'm on my third baby and I'm single and often, it is shit - I am barely able to function a lot of days. But being single, I don't have to go back to work at the moment which is a fucking lifesaver tbh.
I'm often useless with my older children, this is babies for you - they aren't more important than the others, but they are more urgent which means, that yes everyone else has to come second for a short time while they're little - and that is OK. Honestly it is. You have done the intensive stuff with dd already . That is how come she can survive being slightly less catered for right now, iyswim...she has the tools to cope with that and the baby still doesn't, so, your needing to put him first is absolutely right.
DP can totally wait, too...you can recover from something if it is only a few months, and neither of you has been horrid to the other, or done anything hurtful. This is a baby, it is normal.
Much love to you. You sound fantastic and your children and your DP are lucky to have you xxxxxx