Fishing is hard only in beginning
it is easier to focus on the fish rather than the button

if you see the fish swim to left, you push your left control stick
if you see the fish swim to right , you push your right control stick

if you see the camera zoom in, be ready because there are 3 possibilities:
1. if the fish jump, just spam RT
2. if the fish dash to the left, just spam LB
3. if the fish dash to the right , just spam RB

There are some RNG trick
The fish can only swim in a limited area
If the fish in on the left side of Kisara, then the probability of it will swim/dash to right is higher, so push your control stick to right side
If the fish in on the right side of Kisara, then the probability of it will swim/dash to left is higher, so push your control stick to left side

Try to practice on fishing spot in early games, its easier
and getting rod after 10 type of fishes makes things much easier

If you still struggle, use speed hack to slow down the game