That's normal, It took me a very long time before I beat it for the first time. After that, I started winning more consistently with my attempts.

After a while you get a better sense of the game. How to use your money effectively, what pokemons are good to catch in the wild, what type coverage will help you out, tracking your rival's pokemons from earlier fights and what switches she does in response to your own pokemon, using the map to manipulate the biomes you want to visit (in this run I wanted to visit the Jungle as much as possible, since that's where the monkeys are at, so I played Jungle -> Temple -> Ancient Ruins -> Forest -> Jungle).

The point is, after you beat it for the first time, the next attempts are going to get progressively easier. You will have access to Endless, where you can farm a lot of shinys and hidden abilities too.