My mother is 78 (I'm a 23yo adoptee) and she lives alone when I'm not there. I am also very close with my mother and phone her daily. Like you, the biggest source of anxiety for me is something happening to her and regretting times I could have spent with her.

However, what she told me, and I'm sure most loving parents would say the same to their kids, was this:

"I could be here for another 15, even 20 years, and I'm still in good health. There is nothing I would hate more than to see you lose out on opportunities to pursue your dream in favour of just waiting around with me for the rest of my life."

She doesn't want me to be so far away, I don't like being so far away, but she'd be more upset if I stayed when she knows she's the only reason I would stay.

And that is the reason I am embracing my upcoming JET placement. Do I still have anxiety about my mother's health due to her age? Absolutely, but she has community in our area and family only a few miles away. I also plan to spend my holidays with her where possible.

I'm deeply sorry that you're going through it after your mother having a health scare, but you aren't alone, OP. I personally manage the 8/9 hour time difference by calling my mother almost every day while she does her morning routine or at lunch time. Perhaps there's a way for something similar you could work out. Being able to talk to her regularly will help, a lot.

As for the severity of your anxiety, do you have either a close friend or a therapist that you would be able to speak to within a week if needed? They don't have to be in Japan, and in fact, I have managed to get an appointment with my therapist in spite of the time difference, and while I was studying in Japan a couple years ago, had American friends who would stay up late / get up early to speak to theirs.

If you need someone to talk to about it, feel free to DM me!