You are 'back' to being stressed/overwhelmed/failing... just a coincidence your partner is 'back' at the same time?
I want you to keep posting because it might be a way of staying connected to the outside world, even if it is only virtual.
You have done an extraordinary work staying sober 2 years. It does not look you had the best circumstance to reach that. You know better than anybody else you can destroy that in seconds. You also know you are struggling and vulnerable.
I can't emphasize enough how vulnerable you are at the moment. I don't know why you say you are 'failing'. But if you are not drinking/using whatever you are clean from and your kids are fed, you are excelling at life.
The only obvious mistake (and it is a massive one because of the many dangers) is staying in the same environment/house as your partner. It is incredibly bad for your children and it is bad for you.
Your anxiety can be the result of this very stressful situation and the hard emotional and physical process that comes with post-birth and bringing up two very little ones. It can also be more serious and you may be suffering from post natal depression in whatever form.
Wherever you are I guess you have some contact with midwives. Would you be able to confide in one? Let them know about your anxiety (or even your partner's behaviour)? The three of you deserve protection from him.
He can't be anything but bad news unless he decides to sort himself out. Even in that scenario, you can't cope with the ups and downs of someone else's early days in sobriety. He should be your rock now. He should be taking as much responsibility/work for his kids as you are or more to help you recover. No matter what the reason is, his behaviour is a massive betrayal and something to shelter yourself and your son and daughter from.
Get those keys and go to your parents' house. No matter how exhausted you are, you will be more exhausted if you stay. Talk to your midwife/doctor/a friend. Let someone else take a bit of burden from your shoulders. Look at your kids and your incredible achievement over the past 2 years and give yourself a massive round of applause.