Are you usually a perfectionist at work? The other thing I've seen happen is perfectionists working at 110% then in times of stress dropping to, say, 90%. Everyone else works at 90% and don't get criticised but the person who's dropped from overperforming to performing averagely is pulled up on it.
If there's a way to inform work, ideally through hr that your mh isn't great I would do so as it puts an obligation on work to support you rather than haul you over the coals.
With regards to the presents, maybe stress less about them being "right". Do you know the 80/20 rule? As a perfectionist myself it's one of the best things I've learned to adopt.
I also think your 12 year old is old enough to understand that her attitude isn't always kind and unkindness hurts adults too. If it's ungratitude you're getting from your two eldest, well my parents would have responded with consequences and I think as a parenting style it's still valid.
Happy to come up with some present ideas if it won't overwhelm you (just let us know budget and favourite craft activity). Alternatively, ask your eldest two to help choose the presents you get their sister. Theyll like being involved and may give you some really great ideas! Actually, youre 2yo may enjoy this too :)
Im not going to tell you to stop worrying about the the SEN diagnoses as I know it isn't something you can switch off and it must so difficult dealing with 2 children going through this process simultaneously.
What I will say is that there's an adjustment period with any long-term diagnosis and it can feel a bit shock to go from something being expected to confirmed. It sounds like they're both getting appropriate support though which is brilliant - as you adapt to the diagnoses, you will all 3 learn more about the conditions and how to live your best lives with them. Ultimately what you learn will lead to skills that make life easier. But it's ok to grieve the loss of the lives you wanted for them. They will both be fine because they clearly have a mother who will do everything she can to support them and make their conditions easier to manage. They will learn such resilience and your pride in them will increase with every passing year.
Don't forget to save some pride for yourself too. You come across as close knit family so use that - it's a strength :)
how much support do you have - friends, partner or family? It's time to lean on them if you can.
Don't stress about the house too much - just do what you can because the less stress you put yourself under now, the quicker you'll bounce back and once you've bounced back you can deal with any domestic chores you've been putting off.
Focus right now on the essentials and in making your daily life as easy as possible. You are allowed to put one or two spinning plates down for a while! They won't break :)
Your focus right now should be in 3 areas, in no particular order: your wellbeing, your children's well being as they adjust to the diagnoses and keeping your head above water at work. Let go of everything else for now.
You've got this.