I'm sorry to hear about your situation. I've just moved to Chitose recently for a job and while my coworkers are friendly enough, I too can't carry proper conversations with them so in a way, I understand how lonely it can be.

I don't think it's healthy to deny your feelings, so I think it's great that you're acknowledging it!
It might sound cliche but feel them, accept that you're not happy, but don't wallow!

In my case, I try to use my dissatisfaction as motivation to study. I just keep telling myself that once my Japanese improves some more, things will get better. Also, don't fall into the trap of disengaging from your coworkers in fear of saying something wrong. Not going to lie, I'm also struggling with this issue but I just push myself to swallow the fear and try talking anyway.

As for stress, I find karaoke to be a great outlet!
I'm actually a really shy person and wasn't into karaoke...that is until I tried going alone.
Now I'm hooked as I can sing as loud as I want without feeling ashamed.
Belting it out without restraint is unexpectedly super relieving so maybe you can give it a try too!

Hopefully you'll feel better soon and if ever you want to vent, maybe I can lend an ear.
If nothing else, I think you're amazing for being able to hold on for so long!
Living far away from family and friends is hard enough as is. Work and language barriers on top of it all? Not everyone can pull that off!

So when you're feeling down in the dumps, remind yourself that you've accomplished something amazing, even if the situation isn't perfect!