I have the same problem!! I typically try to back track and find the root cause of my insecurities/bad thoughts and I try to find (if any) evidence proving my thoughts are right. I also like to verbalize what I’m feeling to help me realize how small these thoughts truly are. Taking a step back and looking at all the feelings helps a lot
Heal yourself so you can come into the relationship whole and not looking for someone else to complete you. If someone is going to hurt you it is not your fault or within your control. There is nothing you can do or not to do change it. Appreciate and cherish the relationship moment by moment.
She asked for advice on feeling secure in a relationship. The relationship will not feel secure until she is secure within herself. It didn't sound like being triggered once or twice it sounds like its a pretty repetitive or continueous struggle to feel secure. That is a case in which they are not likely healed enough from the past to be in a healthy relationship now.