because I did not want to escalate the situation further so I let it go.
No, you didn't. Not in the way "let it go" means to release the feelings and move on with your life. The ability to examine and release your feelings is an important one in mindfulness. You don't deny or bury those feelings. You examine them, deeply I'd hope, extract the learning you wish to from the examination, then you move on to the next experiences of your life.
You need to deep-dive into why you have those feelings. Analyze them as dispassionately as you can, and try to figure out where they're coming from.
One of the techniques I've used to work through some difficult thoughts is the "Five whys." It's pretty simple to describe, and I'll throw out some pointers that have helped me. The technique is just to ask why five times in a row. So, when you say, for instance, "It hurts me so much that..." ask yourself why. Then hold that answer up and pick it apart. Find some statement you made in that, then ask why again. Then repeat until you've asked the question five times. So, let me invent some answers for you, just to demonstrate how it might help
"It hurts me so much that someone I have trusted for so long could do that and lie to my face about it."
WHY
"Because trust is important to any relationship"
(record scratch!) Actually, let me stop here. You have to make the answer about you, and especially about your feelings. These are feelings, not facts, that you're investigating. And feelings are your own creation, not facts about the world or statements of truth. Let's start over.
"It hurts me so much that someone I have trusted for so long could do that and lie to my face about it."
WHY
"Because it hurts if I think they don't value me."
(ok, that's more honest, and personal)
WHY
"Because I care that people consider me a valuable relationship"
WHY
"Because..."
Now, obviously, I'm just making this up, though these particular answers are similar to ones I've given myself. I'm just trying to demonstrate the technique. I'll warn you it can be painful. Those last two whys can be hard to get real on. They might feel forced. But try it anyway. Sometimes, the answers aren't flattering. You might, let's admit, be pretty fucked up. Recognize that. Let that happen, without judging. We're all flawed, so don't be afraid to understand your flaws. Understand them.
Get to know yourself. Deeply. It'll make life easier, even when it hurts to go through the exercise.