I just can't find the perfect job for me. Currently i have a lvl 17 thief, lvl 15 merc and 13 MA, but i don't know what class is for me the best. I really like fast combat and high speed stuff. could anyone help me out?

Thief is the fast class, high dodge+fast atk. Especially with atk speed buff. But you might want to get merc up to thirty for its shield. And you might want to get atk skills from the blade class to kill faster.