have (your) heart set on (something/doing something)

set (your) heart on (something/doing something) - to decide to achieve something.

John had his heart set on becoming a doctor.

• be as silent as the grave
• be as slippery as an eel
• be as snug as a bug in a rug
• be as sober as a judge
• be as solid as a rock
• be as sound as a bell
• be as sound as a dollar
• be as stiff as a board
• be as stiff as a ramrod
• be as straight as a die
• be as straight as a ramrod
• be as strong as an ox
• be as stubborn as a mule
• be a star in the making
• be as thick as thieves
• be as thick as two short planks
• be as thin as all get out
• be as thin as a rake
• be as thin as a stick
• beast of burden
• be as tough as old boots
• be as ugly as sin
• be a sure thing
• be as white as a sheet
• be as white as snow