Hi Op
I think you are just honest, maybe too honest for mumsnet,
I don't think even good enough mother's are perfect,
I think all mothers have times in which they might find their children a bit irritating for e.g if mother is exhausted , it's time of the month feeling like shit ect,
I think self reflection is a good thing,
As least you are aware of this,
So you can act on this, according,
Such as if you feeling like this it may be a sign of vitamin deficiency,?
It could be you are not having enough support in your life in regards of bringing up your daughter ?
This feeling of feeling of disconnect feeling in regards of oldest child to youngest child feeling more of a mum now to youngest and finding oldest irritating,
Could be just simply a reflection of your oldest child's Age , for e.g if your child is a teenager, ect
Then it's understandable you will encounter experience various different challenges with each Age stages of any child you have,
Which is typical behaviour challenges of that age range for a child,
Also this disconnect feeling you are currently experiencing with your oldest and youngest children,
Could a reflection a manifesting of the kind of family dynamics you grew up in?
Did you have OK childhood or was it a confusing mixed bag, of childhood or Crap childhood?
Have a think about all the points I have brought up @ChiChaNaYubi
It could be a mix of most of these points or some of them?
If its mixed up family childhood dynamics you experienced ,
Have a reflection whether going into having a bit /some therapy would be beneficial for you too,
To look at this and address what your feelings are trying to tell you