It's the constant getting to work and drop off and pick ups in different locations that I feel is tipping me over the edge.
I would suggest it is that and then more again.
You work part-time. In many respects, I think part-time work in a skilled field can be more stressful than a full time job because there is no working week "rhythm" and you cannot establish systems for dealing with day-to-day household and childcare concerns (the "business" case for paid-for household help is lessened because you work part-time etc). A typical week can be "bitty" and always rather "half and half". This, in itself, can be very stressful as you may have a different routine for each day of the week, which means you can;t "automate" your life.
You have to be reasonable and quite cut-throat about what is actually manageable. When I was sinking (badly), I started to ask other working women of my age how they did it. It was surprising what people admitted to on the side. Almost everyone I asked had a cleaner, sent out their ironing, and paid someone to tidy their garden every month. One woman paid her retired mother to cook her family meals twice a week.
It made me realise I was just trying to do too much, and it was killing me. I was so worn down, I even contracted pneumonia, for cyring out loud. I used to wake up on saturday mornings with a "to-do" list downloading in my head, and just feeling near to tears.
The situation got so bad that DH and I ended up at counselling and, in one session, I just exploded 
. We got very close to splitting up: not because I didn't love him, but because I just couldn't cope with the circumstances of my everyday life and I didn't know how to make anything change at the time.
I'm afraid my situation did not properly resolve until a set of rather horrific personal circumstances meant that I voluntarily left my job with a substantial payout (it wasn't an antagonistic resignation, and was fully supported by my very understanding employers).
I mention this because what was truly extraordinary was that over the following three months after leaving my job, I physically felt years of stress and strain "drain" out of my body (it was the oddest feeling, as though my finger tips were dripping out toxins). I had no idea, until that point, just how stressed I was on a permanent basis.
Now I see things with clearer eyes. If you look at modern life through an evolutionary lens, all those moments of stress in a day are similar to your distant ancestor being confronted by a lion.
That stress when you are stuck in traffic and have to pick up your child from school at a certain time? That's a lion. Walking into your house at the end of the day, knowing a meal needs making, bags need sorting, washing needs to go on? That's a lion. An emotionally fraught conversation with a sibling about something bad happening in her life? That's a lion. A shitty email from a client about some work you did, coinciding with worries about keeping your job? That's a really big lion with nasty teeth. Anything that provokes a stress response, those spidery feelings in your gut, or even just a feeling that is not joy, happiness or contentment is a version of that lion.
So when you count them all up, how many lions do you confront a day? Because your distant ancestor probably came across a lion a week at the very most. Yet modern life means we experience maybe ten or more "lions" a day.
Our bodies and minds are just not designed to cope with this level of continual stress. Yet hardly anyone talks about this angle. I had three bouts of therapy when I was bad, and it did help to talk, but the problem was always about the lions and it wasn't until I got rid of those lions that I actually got better.