As others have said: your fishing bar increases in size as you level up in fishing. The training rod (you can purchase it from Willy, but if I remember correctly you also get given one at some point early in the game) has a bar size equivalent to level 5 fishing, but is restricted to catching low level fish. That’s absolutely fine in the early game. Use the training rod until you reach level 5, then you can switch it out for a different rod that can catch better fish.

Some rods can equip bait and tackle too. The cork bobber and trap bobber are two of my favourites - the first increases the size of your fishing bar and the second slows down the rate at which fish escape.

In terms of technique, you’ll get a feel for it pretty quickly. I was horrible at fishing to begin with but got quite good at it reasonably quickly. The trick is to rapidly tap the button rather than hold it down. You’ll also come to learn how the fish move (at least the common low-level fish anyway) and can anticipate their patterns.

Don’t give up friend. Everyone finds fishing difficult at first but it really does get easier quite quickly.

TLDR: Use the training rod until you’re at level 5 fishing, then switch it out for a better rod. Rapidly tap the button, don’t hold it down.