The best possible advice I can give is take your time.
There is no rush or hurry to get it over with - none. You are IN NO WAY imposing upon anybody, so remember that. This is their #1 priority, so don't worry about the time, how long its taken, or anything else at all. Take all the time you need, and try not to even think about it.
This may be a difficult one: try, if you can, to trust your interviewers.
They will be professionals who are well trained to handle this situation with empathy, care, patience, and kindness. They will believe you. They are there for you. They know that this is a very difficult thing to do, and they will be understanding. And they CARE. They really do care.
And if there is anything you are nervous or uncomfortable about, do mention it. At least ask - there is no harm done. It may not always be possible to accommodate your request, but then again it may. And if the interviewers know that you're uncomfortable but cannot meet your exact request, there may be something they can do to at least ease your discomfort.
Go to the toilet before you go in, because you can't tell how long you'll be in. And do try to eat a little something before you leave for the station, so that you're not hungry, but not uncomfortably full, during the interview.
Take your time. Think about your answers if you need to.
If you need to cry, go ahead and take a moment.
If you need a break, don't hesitate to ask for a break.
Don't worry about anybody but yourself in there.
Be as honest as you can. If you don't know or aren't sure, just say so. If you think it could be either this or that, just say so.
Ask them questions, too. Ask anything you want to before it begins to get your bearings, but also ask questions during the interview if you need to, as well. Ask ANY questions.
Try not to be embarrassed or leave out details for the sake of others in the room. They'll have heard it all already anyway.
Remember, you are strong. You may not feel it, but you ARE. You're here, aren't you? You've come this far! You can do this.