I was a really easy to scare child.
I remember being like 6 or 7 and being really afraid to swim to the far end of the pool alone while taking swimming lessons, because this Yu-Gi-Oh card would always pop into my head.
Also, does anyone else remembers this final boss from a shmup called Strikers 1945 2? I recently discovered it was not some weird nightmare I had as a kid.
What really got me in Super Metroid was Crocomire. Just wandering around Norfair, opened an ordinary-looking super missile door, and suddenly I was trapped in a hall with this giant multi-eyed melty-skinned thing . I had no idea how to defeat this thing, didn't know that charged shots could hurt bosses, so once I ran out of missiles and super missiles, all I could do was watch as it slowly backed me into a spiked wall and crushed me to death.
YES! Super Metroid was the first game I ever played from beginning to end and beat. I was really young and there were were lots of times that the boss fights were just too scary and stressful. Crocomire was one of those bosses that I got stuck on for a couple of days because, like you, I just didn't understand and couldn't aim lol.