OP, I'm so sorry it's all so crap, it's completely understandable you feel so low, it's a lot to deal with, and I'm sorry about your mum.
Do you have anyone else who can support you, a friend you can regularly meet up with? Siblings? Your dad?
Counselling might help you process your feelings, your GP should be able to arrange this.
You've a lot to deal with and occasions like mothers day are difficult if you feel burnt out and unappreciated.
It sounds like you are doing an amazing job.
You have 3 lovely DC.
Your ex sounds awful and his new partner does too.
Make sure you get everything you can from the divorce, get a good solicitor, ensure he pays everything he needs to and go via CMS, court order if needed.
His new life may not be as rosy as it appears, his new carefree life is coming to an end and he may end up treating his new partner like he has treated you.
Reassure your 10 year old, of course it will hurt, your DC might feel like they are being replaced but hopefully longer term a new sibling will bring joy. Their dad should be reassuring them on this point but clearly is unable/ unwilling.
When youngest DC goes to their dad's do something for yourself, the older two are OK to be left alone for a bit.
Try and distance yourself emotionally from your ex and be kind to yourself 💐