Trust, like drinking water, is a commodity one can’t live without for very long.
Trust is something we resort to constantly without paying much attention to it.  In day to day life we have evolved into putting trust on automatic; in effect trusting “trust”.  Most of the time it works out.
We trust that the car in ongoing traffic will honor their signal and not turn in front of us.  We trust that the barista will deliver our order even though we didn’t ask for the receipt.  We trust that the valet will return our vehicle and not drive it to a chop shop in San Bernardino.
We trust our spouse, our children, our close friends, as well as our new friends with unproven track records.
We trust that the individuals that we talk to in the course of a day are telling us the truth.  When we become suspicious, we feel bad; we feel it’s a failing of ours not to be trusting.
When trust becomes broken, either sharply, or worn away over time, it’s a big deal.  A valuable commodity has been tainted or even destroyed forever.
It’s like a product recall from a favorite company–suddenly that innocent, everyday purchase morphs into something undesirable, ineffective or even deadly.  
Of course, when that undesirable, ineffective or dangerous thing is someone you used to call “friend”, it’s a lot bigger deal than having to bring your Camry in for a new wiper switch.
Trust itself gets a bad name in a less than honest environment.  “Being trusting” has come to mean “Being duped” in the same way that many common words have been supplanted in common usage by their lesser, more compromised definitions.
While the act of moving once trusted friends into the category of “just another acquaintance” or “enemy” sticks prominently in our consciousness, the thing we really get stuck in is the fact of the betrayal.
All we can think about is “how could they?”
But the thing we ought to be thinking about is what opposite behavior we can deploy, so as to answer untrustworthiness in kind with the thing we can be responsible for.
Making oneself worthy of trust, as a practical matter, is the best safe haven for being betrayed.  Double down on that, rather than continue to grind your teeth and curse your erstwhile friend.
How?
By keeping one’s word, being respectful and considerate of others, providing help when needed, being honest and tolerant of others, setting an example of positive behavior… these are under our control.  Theseare what to focus on.
When we work to become more worthy of other’s trust, we pay things forward and create freedom for ourselves that otherwise might have been gobbled up by resentment.
Lost some trust?  Gain it back by taking steps to trust in yourself.