I suggest using emotional wounds. Here's a list for inspiration (there's also a book, and it even has a section of wounds aptly named "Misplaced Trust and Betrayals." I highly recommend it.)
Give the protagonist a suitable wound and then have the antagonist hurt them just the same way again. The past and the present will deepen the hurt even more.
Or, if you already have a specific type of betrayal in mind, make it the second time around for the protagonist by having them being betrayed the same or a similar way in the past.
Dig where you stand
To get at the emotional writing of the protagonist's reaction to the betrayal, I think there's only one possibility. You need to dredge up a betrayal from your own past. Yes, it will hurt... writing is sometimes painful, but that's the cost of good writing (much like a lot of other forms of art). You need to expose your soul here.
The good news is, it doesn't have to be a betrayal similar to the one in the story. It's after all the feeling of being betrayed that is important, not the betrayal itself.
Come to think of it, this is also true when it comes to the emotional wound of your protagonist. Having been hurt once and now being hurt again but in a different way would of course still pack a double punch.
The most important reaction from your protagonist should be visceral. I.e. in the body.
What emotion does the betrayal cause? Rage? Terror? Devastation? Or something else? All of them?
Where does the pain of the betrayal sit in the body? What does it do to the throat? The stomach? The head? The sense of balance? The heartbeat? The chest? Etc.
You might have to go through the whole body until you find the right place and the right visceral reaction.
Do only one or a very limited number of reactions!
Find the one telling detail that will show your protagonist's visceral reaction.
This is seasoning, and no seasoning makes the story bland, while too much seasoning makes it inedible...
On the other hand, this is probably a key moment in your story and your protagonist's life, so you can pepper it more than any random scene.
Fix it in editing
Is this the first draft of your story? Then do the best you can with this scene now and finish the first draft. Put it away for a while (from a week to a year, depending on how quickly you forget or relax about your story, or how long you can manage without working on it... ;-)
Pick it up again and read it like you read any other book.
Maybe that scene won't even stick out? Or maybe it does, then see what "the boys in the basement" (your unconscious in Stephen-King-speak) have been up to for all this time... maybe they've come up with a better way to write the scene? Then change the scene!