I need some SERIOUS help today!!!

After last night, listened to my husband involve his kids on speakerphone (they are 11 and 16)....they were all laughing at me. He was saying "Well, we all decided (meaning him and his kids), that I was not welcome over until my attitude changed. Then, he changed it and said "well, they need stuff to pack for lunch tomorrow, bread, peanut butter, etc and since I have "not been doing my job as a stepmom and shopping, etc and taking care of my stepkids"....I need to bring it over NOW!!!!
He continued with his same old diagalog, I abandoned them, I am very sick and bipolar and psycho and I need help, etc. I can't believe someone is so sick that they bring their young kids in on this type of stuff.
So, I caved yesterday.....when he called and was on his kids' phone, I answered it and started to engage in the conversation. He threatens me again and makes me feel bad "We don't have any food, I am not doing my responsibilities, etc" and that if I dont start acting right, he is going on a date and filing for divorce.
I KNOW I MUST BREAK OUT OF THIS PATTERN. I hope you guys don't get mad....this has been the cycle for many, many months. I have been in and out of this marriage so many times. I am really confused at times if I AM REALLY THE ONE THAT NEEDS HELP HERE!!! Then, the other problem is I am very lonely and isolated.
What do you guys do when it is late, and you are just dying inside, terrified of being alone and can't make it stop. What can I do different today so I maintain complete NC and start healing from all of this.
Yes, I felt bad and went to the store and got the kids stuff for lunch, of course making matters much worse....
I just don't know how to get the courage and strength to do this when I am so beaten down right now. I really was not always like this. .......it is truly a NIGHTMARE!!!!