堅持不懈

to persevere unremittingly (idiom); to keep going until the end

strong / solid / firm / unyielding / resolute

to persevere with / to persist in / to insist on

to hold / to grasp / to support / to maintain / to persevere / to manage / to run (i.e. administer) / to control

no; not so / (bound form) not; un-

untiring / unremitting / indefatigable

lax / negligent