I was struck by the same thing as bert, where STM wrote "It always feels very wrong to sing a song in a different key to the one I originally heard," and bert commented that it sounded as if she must have perfect pitch.

I can never tell if a song is being sung in a different key or off-key, unless I'm trying to play an accompaniment to it. I'm impressed by anyone who can.

So definitely don't give up. Lots of good advice in this thread.

The other thing that struck me was where STM wrote "...come from a non-musical family. We've never listened to music nor played any instrument and no one, except me has taken an interest in it. My school did not sing any songs..."

Singing is something we are built to learn the same way we learn talking, and maybe even earlier. Any family and any school which doesn't encourage children to sing is really letting them down pretty badly. In fact in it's way, it's a form of malnutrition.