How do I pinpoint a trigger so I know what problem I need to work through?
Thanks for your question and for reaching out. Sounds like you had a meaningful and healing time with your sisters talking about the past and in particular, your parents divorce. As I have not met with you, the following are my thoughts in general about your question.
First, I would not assume or be surprised if your reaction after returning home - irritable, anxious and tearful, is related to the events you discussed with your sisters. You wrote that you "worked through" the content of what you and your sisters discussed. I don't doubt that you worked on this in past therapy. However, past loss, pain and trauma (if any) do not necessarily "go away", especially if your conversation with your sisters reminded/brought up some of your feelings about what you and your family went through. In other words, processing feelings do not mean they are "gone." It seems like you were angry, sad and anxious after the talk with your sisters. If so, this is "normal" and understandable. Dealing with feelings is sometimes a process of working them through and then needing to do so again when those feelings return.
Secondly, I think it is important to give yourself some "grace" here. You are human like the rest of us, and human beings have feelings. However, as humans, our feelings sometimes "come and go." So please, don't be anxious about feeling as you do. Like most emotions, the intensity of your feelings will pass. It is important to have faith in yourself that you will be okay - that this too shall pass.
Finally, I would suggest not worrying about "why" you were triggered. We humans usually want to know the "why." I think it is important to remember that even when we don't know "why", it does not mean that we can't handle what is going on. It just means, at the time, you don't know what is going on. In all probability, even when you don't know the "why," your feelings will pass.
Caring for five children under eight years old sounds challenging, particularly when you are triggered.
Wishing you all the best. Please take care of yourself.
Steve