I am not happy so far...
The leveling system is so bad, i can't make the character i want. Even the standard class are so bad too.I don't know how to enjoy this game anymore
You're not alone in being frustrated by it. It's the single biggest complaint about the game.
Some people might disagree, but I think it's working as it was intended to. Oblivion is designed to get harder as you level up. This was a response to complaints about Morrowind being too easy at high level.
One simple solution is to move the "difficulty slider" to the left. That slider only adjusts relative damage, so it's useful for balancing combat without changing anything else in the game. I find that for some kinds of characters, the game seems more "realistic" with the slider set at around 35-40% rather than the default 50%, especially at higher levels.
The second solution involves learning to use the tools that the game provides. Alchemy is very powerful. You can do almost anything with Alchemy that you can do with all the other magic types, and poisons are very effective when you get Alchemy to high level. Also, you have to keep updating your spells and your equipment as you level up. You can't keep trying to heal yourself with that novice spell, for instance.
By the time you get to level 6 or 7, you should be ready to face enemies who can reflect damage back on you, and who can heal themselves. It gets worse as you go up from there. Some of them are immune to certain types of magic, or to poison, so you have to know who you're fighting, and what to use against them. As you get into the teens, enemies will have a lot more ways of attacking, and they will start reflecting spells back at you.
The best plan I've found is to focus on one good ranged attack (like Marksman or Destruction), one good up-close attack (like Blade or Absorb Health), and one good defense (one armor type, if worn, or Alteration for shield spells.) Other skills like Block and Acrobatics make good majors, because they advance slowly. Then intentionally train two or three Minor Skills that are useful to the character: Sneak, Security, and Armorer are good.
If you do that, you should be getting +3s or +4s as multipliers, so you should be keeping up with enemy advancement, as long as you keep your spells and equipment up to date.