I am sorry Art, I know it feels like the end of the world. My parents w a 'stay together for the sake of the kid's ' couple and it was horrible.

But this is different.
Marriage was created so fathers could ensure their daughters were looked after. In a time when the average life expectancy was about 35.
The thing is humans never stop growing, changing,needing,wanting. What you want at 17 isn't what you want at 25, then again that may change again when your in your late 30's early 40's.

Your folks are good people. I think you should probably talk to them. Quite often the things we make up in our head is far worse than the reality of it.
Your folks sound, determined. So whatever is done they won't have made the decision lightly.

It could well be that your dad is just very stressed and not sleeping well. So your mum is making her own space.
Or perhaps it's something else. As scary as it feels now, I can promise you, two happy parents apart, are a thousand times better than two miserable people together.

I'll be thinking of you, take care, be strong and have faith.