The key element in my personal experience is to make peace with how things are in that moment. That something triggered this unpleasant emotional state/disturbance, that it is totally okay, and not to push it away but instead give it gentle, loving awareness. The trick that works for me is to think of someone who I love, or someone who loves me, imagine that we look each other in the eyes or gently touch the part of the body that feels unpleasant, feel the "warmth of love" developing in me, and then switch from using a third party to myself, so both the sender and recipient of love is me. Looking into the mirror helps. And then staying present with the sensations, realizing that I can have a clear and calm mind and the unpleasant sensations at the same time, give them the attention they deserve, passively listening to what kind of memories come up (if any), and just wait and give it time.

For example, I might suddenly get hit with a feeling of terrible loneliness, even though I am with my partner or among friends. Instead of telling myself "hey, listen, you are not alone, stop feeling lonely!", I tell myself "this is a memory of a moment where you really were left alone. poor me. thank you for reminding me of that past horrible moment, now that i am fully capable of giving it the loving attention that it deserved back them but didn't get".

This process takes some effort to learn, and accepting that you are still learning and not already perfect at it is a key part of the practice. But even small moments where it "works" in my experience finally actually resolve stored up emotions, so each time an unpleasant emotion comes up it hits me less and less. It is not important to understand exactly what triggered it, or what the past traumatic memory is. Do not ask yourself, just be, and open your heart.

There are many different descriptions of techniques. The one I like the most is "Feeding your demons" by Tsultrim Allione.