That's just fish, man. We cook and freeze them for a reason. It's gross, mentally speaking, but totally safe.

You will usually not see parasites in fish from the store for two reasons. The biggest is that fillets are inspected and have visible parasites removed during processing (because they gross people out), and then other being farmed fish just have a lower chance of having parasites (untrue if farmed in open ocean nets) because they eat pellets and live in a controlled environment.

But otherwise? Fish generally have some worms. Seriously, go fishing and catch a few trout. Process them and then take a close look at the flesh near the tail. You will typically see a few worms once you really look.

It's completely reasonable and normal to be put off and lose your appetite from seeing this, that's exactly why they take these out of the fillets during processing. But just bear in mind that worm is entirely normal to find in commercial grade fish, and while people try hard to remove them all before they hit your plate sometimes a few get through. We already cook fish to deal with this, you're fine.