It goes like this: something triggers a negative emotion - it could be anxiety, panic, fear, anger. That initial reaction is almost instinctual. You go into fight of flight mode and feel the need to instantaneously act on your emotion. But after this initial reaction - it's documented that initial anger lasts 90 seconds, for instance - you have a choice. Yes, initially there is a physical reaction. It's not that your brain is just playing tricks on you. The chemistry in your brain, and therefore your body, temporarily adjusts. However, it's not a permanent adjustment. It's not even a long term yet temporary adjustment. It's a momentary one. After this point, it's up to your own internal interpretation.
Let me give a real life example. Someone from my past used to constantly put me down, telling me I'd never be successful, that I saw the world differently, and that I didn't have "what it takes". Which is, looking back, a rather ridiculous statement - the blanket insult "you'll never be successful" is a little too broad to actually be accurate. But at the time, it was horribly painful. My self-esteem and confidence were at rock bottom, it was the peak of the economic crisis and business wasn't ideal, and I basically believed this person. I let it get to me, and it stayed with me for quite a while. Fast forward a few years. The other day, someone asked me a question about my business strategy. They were just asking a question, trying to learn about my business and wanted to see if they had any helpful insights, simply to be nice. But my brain went straight back to "you'll never be successful". I immediately felt like I was being questioned and attacked, like this person too felt I'd never succeed, and I went on the defensive.
Now that I realize what happened, I can look at it from a more objective point of view. I had this initial "oh no, not again" gut reaction. But instead of assuming the helpful person meant me degradation and (emotional) harm, I could have recognized it for what it was - an instinctive feeling that was caused by myself, and not by the person asking. Even if they had been questioning me, it's my choice to believe them, to take it to heart, to let it eat away at my self esteem. In this case, the damage was double because not only did I unnecessarily upset myself, I upset someone else who was trying to be helpful.
This all said, it's not easy. Instinct is incredibly powerful, especially when it's based on real life experiences. Plus, there's a fine line between not learning from your mistakes and not taking the past out on the present and the future. If I always thought "oh that person doesn't really mean xyz, it's just my interpretation," I could not only miss some important lessons, but I could end up getting hurt (emotionally or physically) by being too naive. It's a delicate balance. I think it comes down to looking internally, knowing your sensitive points, and being aware that those are your own insecurities. When you become aware of these, you can more easily pause a moment when they ignite and ask yourself, "why am I feeling this way?". Another good trick is to objectively reverse the situation. "If I said xyz to so-and-so, and he/she reacted this way, would I think it a reasonable reaction?". Don't allow your sensitivities to play a part in your answer. If that's too difficult, pretend you're not in the situation at all... "If Bob said that to Mary, and she reacted this way...".
I've learned, and am still learning, this lesson the hard way. I've actually asked very trusted people to call me out on it, albeit nicely. I've told them that I'm trying to let go of past hurts, and that when I start to bring those into the present to gently tell me "you're doing that again". If I catch it in the moment and am able to reverse course, I believe eventually I'll be able to stop it from taking place in the first place - or at least I hope so.