Well, tbh I'm not surprised that she feels ashamed - the fact that everyone's first instinct is 'take her to therapy' is probably underlining society's view that there is something woman that doesn't immerse herself completely in her offspring.
Having kids, especially ones with additional needs, is bloody hard work. There is no law to say you have to have a perma-smile attached and find it fulfilling.
Yes, being miserable is awful. Yes, she needs to find something that fulfills her and try to make some changes that will ignite her spark and give her some enjoyment.
But you can't make her into super mum, and nor should anyone try.
Presumably the youngest will be going to school soon, which will give her a chance to rediscover herself for a few hours a day, instead of being x's mum.
24/7 with kids is life-sapping if you aren't particularly that sort of person. It's why many people get back to work and use childcare.
She sounds perfectly normal to me - she's just chosen to work from home with her children, instead of outside the home and using childcare.
Be worried about her happiness, not the state or otherwise of her mothering desire.