My fear? Never giving my wife the life she deserves. She's been wanting kids and we've been trying for years to no avail. We currently don't have insurance and not enough money to get everything checked out, but I'm afraid it's due to my little swimmers. I have no way of verifying this, and no CR (mainly chatbox), you can't help/investigate.
As for darkness, heights, etc. I did a youth lesson once talking about crippling fear vs. solstice in knowing Christ is watching over us. I won't go full on preacher mode b/c I respect all your different beliefs, but the main point still applies.
NOBODY, in my opinion, is afriad of heights or darkness. If I walk you into a room with no windows and ask if your afraid likely you'll say "no". If I then tell you that room is 10,000 stories in the air a little nervousness may set in, but since you can't verify it you won't feel too different. If I walk you into a room with just rails that is 10K stories and make you walk to the edge and look over THEN the fear strikes. As someone said, the fear isn't of heights, but of falling and what will happen as a result. The pain, or as others have stated, the fear of death. It's natural.
As for darkness, it doesn't attack us. It does nothing good or bad. Darkness has no form. No life. Therefore, it is no threat. We're afraid of what may happen to us without us seeing it coming and beeing able to react accordingly. Walking down a country road at night and a car drives by at 80 mph doesn't scare you b/c you see the head lights in plenty of time and can adjust your path to ensure your safety. Same thing happens without the cars lights on, and although you might hear something, it's far more terrifying of a situation, especially if you can't decypher exactly which direction and speed to car is moving.
We're not afriad of heights, darkness, spiders, etc. We're afraid of the unknown. What happens if I fall from this height? Death? Paralysis? What happens if the lights go out and someone enters the place I'm in? Mugging? Death? What happens if the spider bites me? Is it poisonous? Will it lay eggs in my face? What will happen if I never find happiness in life? What does my future hold?
All natural human thoughs I believe. For me, while it'd be absolutely asinine to say I'm not afraid of anything or even that I don't have some of these same thoughts cross my mind, but they're not crippling for the most part.
Psalm 118:6 - "The Lord is on my side, I shall not fear..."
Worse case scenario? I die and go home. I understand that's not everyones belief or mindset though and I respect that.