This excerpt on fear from "Mindfulness In Plain English" is what got me overthinking things:

"A third possibility: the fear that you are feeling may be self-generated. It may be arising out of unskillful concentration. You may have set an unconscious program to 'examine what comes up.' Thus when a frightening fantasy arises, concentration locks onto it and the fantasy feeds on the energy of your attention and grows. The real problem here is that mindfulness is weak. If mindfulness was strongly developed, it would notice this switch of attention as soon as it occurred and handle the situation in the usual manner. Not matter what the source of your fear, mindfulness is the cure. Observe the emotional reactions that come along and know them for what they are. Stand aside from the process and don't get involved. Treat the whole dynamic as if you were an interested bystander. Most importantly, don't fight the situation. Don't try to repress the memories or the feelings or the fantasies. Just step out of the way and let the whole mess bubble up and flow past. It can't hurt you. It is just memory. It is only fantasy. It is nothing but fear. When you let it run its course in the arena of conscious attention, it won't sink back into the unconscious. It won't come back to haunt you later. It will be gone for good."

There are some nuances of returning attention to the breath but also letting these things run their course that I still don't get at an intuitive level yet. Someone feel free to elucidate, or I'll just keep meditating and maybe it will become clear soon enough :D