There are a number of single words that demand an expanded explanation.
Trust is one of them.
"I don't/can't trust you is hurled between parties.
Often there is an overt incident that prompts the comment; that one is easy to identify.
What follows is the depth of the mistrust, the duration of this condition.
Once on record, it hits and stings and stains. It will not go away easily, maybe ever.
To what degree this is explored, especially with relationships that are expected to grow, will define the quality of that relationship.
The relationship as it was known is forever changed.
A new contract must be formed.
Without discussion, this fractured relationship will not heal properly.