Trust doesn't disappear all at once. It gets worn away with every lie; big or small. But once it's gone, everything looks like a lie and the relationship cannot function. It's easier to maintain trust than it is to try to rebuild it out of what little may be left.
Some things, however, are true no matter how hard you might try to block them out, and a lie is always a lie, no matter how prettily told. Some doors, once they’re opened, can never be closed again, just as some trust, once it’s been lost, can never be won back.