First of all, maybe take him to the vet or call a vet. Ask them what to do.

Next please don't say that. It's self-sabotage. You feeling happy has nothing to do with mishaps and accidents. Your anxieties and ptsd are making a connection, where there isn't.

You're allowed to be happy. No consequences.
Cats are just like that. Curious and like protein. I'm assuming there's milk in the cake. One time my sis cat ate a whole piece of a pumpkin sugar pie. He was fine.
But just in case, call a vet, ask for advice. They'll tell you what to do. And monitor him.

Hopefully he'd be fine. And so will you

There is never a moment or time where I don't worry about our cat. I always know where he is and what he is doing and I don't even like the cat. He gives me so much anxiety and stress but my children love him so I don't have a choice

Highly doubt cake will hurt the kitty, you can relax. Seems odd the kitty would even eat the stuff.