Okay, first off I think your view is quite biased because of your current frustration. In fact this game is very carefully balanced. It is "unfair" in the sense that gym leaders have access to better Pokemon, but you have to considering that you're not playing vs a human opponent. If you play vs Reborns AI with even teams, THEN it will be unbalanced - in your favour. So there has to be some sort of advantage for the gym leaders.
"My friend, who brought me to Reborn, said you have switch between more pokemon in Reborn to get through. No, I do not want that." - If you do not "want that", then you can't really complain about the game being unbalanced and unfair. If I wanna win a soccer match but refuse to move around, then I won't win. But you can also limit this by doing some "research" into which good Pokemon are available early on. And regarding the level cap: If you train up multiple Pokemon, you rarely need Common Candy.
I think Reborn doesn't make it a secret that it's rather difficult. But that's all it is - difficult. Not unfair, not unbalanced.
If that is a big problem for you - which seems to be the case - there are some things you can do that do not include mindless grinding:
- as mentioned above: "research" which good Pokemon are available early on. That way you don't have to train up a new team every gym.
- figure out field effects and use them to your advantage
- ask people to trade you better Pokemon
- abuse the fact that you have a human brain and Reborn only has a predictable AI