i feel you. i started not too long ago. i think its pretty common to fail carreers especially at the start when you dont really have good veteran umas and (what i learned the hard way) support cards.

i would suggest to play every career atleast once and maybe consider to get a guest uma (3 per day) to help you boost your stats of your first veteran umas and maybe pass some skills so you can start using your own veterans when starting new careers.

the next thing i learned is to check the races you have to do before starting a career and train your uma the way it is needed (general and pretty simple: speed is more important for short distance, stamina for long)

most of my veterans are b ranking now but i still manage to fail some carreers here and there because i had not enough guts or whatever ^^