One of the common pitfalls of “mindfulness” meditation is that practitioners become very good at noticing sense objects in their body. (More simply: feelings.) Now, if those feelings are negative, that can lead to more anxiety and fear, and if you contact the emotions around a traumatic experience, you can be re-traumatized; the trauma can become worse.
Understand how the mind works. When something bad happens, the mind brings it up again and again as a warning; “This was bad, you should watch out for it.” The mind is trying to be helpful (our minds/brains are not that smart, but they are trying to help).
If, when a negative feeling comes up, you don’t react to it, or you react with warmth/love/security, it weakens. If, on the other hand, you are upset, and you add an additional negative load to it, your mind thinks, “Oh my, this is still a danger, I should bring it up more often and stronger.”
So the key to using meditation to help with trauma and anxiety is to not react, or to react with warmth, love, or indifference.
This means that mindfulness and Vipassana meditation styles should not be used alone. A concentrated mind (from shamatha: concentration on an object like the breath or meditation, or a loving mind, from Metta or something like puppy meditation will react more calmly or even with warmth.
When that happens, the underlying anxiety or trauma weakens. Repeated applications will reduce it a great deal.
This means that you should always do more concentration and loving kindness meditation than Vipassana or mindfulness — at least a 2/1 ratio, and do shamatha and loving-kindness before you do vipassana. I suggest concentration first (breath or mantra are easiest for most people), then do mindfulness or vipassana.
If you contact an emotion you can’t deal with, immediately switch over to concentration, then after a few minutes, go to loving-kindness.