I've liked the space and often read a picture book of the space.

Since I found it's very hard to be an astronaut, I realized my dream was just dream.

But now, I have another dream that Io host my own art gallery one day as I posted yesterday.

When you were a child, what did you want to be in the future?

Space after comma

When I was a child, I wanted to be an astronaut.

Comma after "When [subject] [verb] [additional information]"

I've liked the space and often read a picture book of theabout space.

If you read one book multiple times your sentence is correct
If you read multiple books about space, it would be "picture books" instead of "a picture book"

Since I found out that it's very hard to become an astronaut, I realized that my dream was a just dream.

"Found out that" means to discover, learn something for the first time
"That" after "realize" because you add information that could be a standalone sentence

But now, I have another dream: that Io host my own art gallery one day, as I posted yesterday.

Colon (:) to introduce something (in this case, your dream)
Or you could write "I have another dream of hosting"

I've liked the loved hearing about outer space and often read a picture book of theabout space.

Since I found out it's very hard to be an astronaut, I realized a dream was just a dream.

But now, I have another dream that I will host my own art gallery one day, as I posted yesterday.

When I was a child, I wanted to be an astronaut.

I've liked the space and often read a picture books of the space.

"Space" doesn't need "the" when you use it to mean "宇宙."

Since I found it's very hard to become an astronaut, I realized my dream was just a dream.

But now, I have a dream to host my own art gallery one day, as I mentioned yesterday.¶
But now I have another dream and that Iis to host my own art gallery one day, as I postmentioned yesterday.