When I was a child, I often had a dream that I jumped down from a very high staircase, and then I would wake up because of thea sense of falling.

I think it might behave been true because at some point in my life, I never dreamed it againstopped dreaming it.

Perhaps, I only dream once a week?.

I only have dreams if I'm very tired or stressed, and the dreams are usually about missing the train, not being able to find something, or missing a deadline for work.

Very rarely do I have dreams about friends I will never meet again, which always leaves a bittersweet taste in my mouth.

It might be the last time that I dream about them;. I always have this feeling when I wake up, so I write down the dream.

I don't know from which momstarting whent, but I seldom dream when I sleep now.

Very rarely do, I have dreams about friends that I will never meet again, which always leave a bittersweet taste in my mouth.

I'm actually not sure about this one, it might be "whom" but something needs to go there

ItThis might be the last time I dream about them; I always have this feeling when I wake up, so I write down the dream.

When I was a child, I often had a dreamed that I jumped down from a very high staircase, then Iand would wake up because of the sense of falling.

'dreamt' is also correct. "Had a dream that ..." isn't wrong but doesn't sound as good

People said it was because I was growing taller; the pressure on themy bones caused the dream.

using 'the' here is acceptable but sounds odd. Usually you use the possessive article when referring to people's body parts, e.g. 'my bones', 'their eyes'.

I think ithat might be true, because at somefter a certain point in my life, I never dreamed it again.

'this' or 'that' are acceptable (and don't really mean anything different in this case) but 'it' doesn't sound natural. 'It' works when referring to previously mentioned nouns, but when referring to broader ideas or whole clauses/sentences it's better to use 'this' or 'that'.

You can also say 'past a certain point', or 'at some point, I stopped having that dream' etc. I can't exactly explain why, but it sounds wrong otherwise.

I don't know from which moment, but I seldom dream when IFor a while now—I don't remember how long—I haven't been dreaming in my sleep now.

'I don't know from which moment' sounds unnatural. Unfortunately there doesn't seem to be a pithy, simple phrase like this to express this in English!
'I seldom dream when I sleep now' is ok, it's just I had to reformulate it to make it work without the first part. Here are a few more options:

I don't know when this started happening, but for a while now I have not been having any dreams. (slightly clunky)
I seldom dream these days; I don't remember how long it has been like this. (also clunky)
Since when, I don't remember, but I seldom have dreams nowadays. (Sounds a bit pretentious)

Very rRarely do, I have dreams about friends I will never meet again, which always leaves a bittersweet taste in my mouth.

'Very rarely do I' is correct syntax and grammar, but in this context it doesn't work well communicatively because it puts emphasis on 'very rarely' as the central focus of the sentence, implying that the rareness is the main new information and makes it seem like the 'dreams about friends (...)' are something that has already been mentioned, leading to confusion.
'leave' or 'leaves' both work