Sorry to hear things are not going well, and very sad to hear about your cat. Pets are so special and it's not fair they don't get to stay around forever, I know that feel. At least you can think you gave your cat a good home for its entire life and that's very important :)
It's also important to maintain a good balance between work and life, and your "work" here is game development. Start small, like go for a walk each morning and evening, you could consider that your "commute" so you start working on games after your morning walk and you stop working after your evening work. Take a break in the evening and do something you enjoy, whether that's seeing friends or playing games or just lying on the couch watching shows.
Try and work a little exercise (cardio or weights or whatever you prefer, it doesn't matter) and meditation into your routine, they will help your brain produce better chemicals which will keep your mood more steady and at least help a bit with depression.
I hope that helps. But overall, do look after yourself. Your personal health is more important than video games or random people on the internet like us.