I've been where you are at and my heart goes out to you. I remember the depths of that pain and how devestation it felt to realize that there wasn't anything strong enough to make me want to stay, including my own children. Because there is a period of arguing with yourself not to do it, and your own kids do come up.
As far as having the conversation with your children and therapist about your trauma, how old are your children? And have you talked over what exactly it is that you want to tell them? How much you tell them depends on their age, and what you tell them is up to you. I think it is something you might want to rehearse over with your therapist a couple times before they come in and prepare yourself for questions they might have. Your children watch you struggle every day, and they deserve to know why there are patches of hard times for you. I think it would be beneficial at the very least for a therapist to explain depression and ptsd, what it looks like, what they can do, and ways they can take care of themselves so it lessens the impact it has on them.