Hang in there, kiddo. Do the best you can with your cardinal tetra and things will work out. When I get in tough situations, dealing with worries, anxiety, etc. I try to focus on what I can control, and just make sure I'm doing the next right thing. Whether it's doing some homework, reading, relaxing with a book (important for my well-being) or whatever, just do that and let the rest go.
I have a tendency to worry about all kinds of things that I really can't do anything about. When I boil down my thoughts to focusing on whatever is right in front of me, the weight on my back doesn't seem so heavy.
Enjoy those fish in the meantime- they are very calming. You're doing the best you can, and they'll be ok.