Considering that Earth is just one of the countless stars in the universe, all the problems we have seems to be trivial.

"Countless" here sounds more correct to me without the definite article, although there are situations where it would be necessary as part of a construction, like "all of the countless stars."

"All of the X": if X is a plural, the verb needs to be for a plural.
Singular: All of the sugar [not countable, singular] was gone.
Plural: All of the children [countable, plural] were gone.

However, some people would say, "You are just thinking about an ideal situation,; you can think that way because you live in a peaceful place and you are well educated, but I cannot because I am always starving and did not even go to primary school."

Every time I think about that, I feel sorry for them.

"Every" is generally a separate word, except for "everyone" and "everyday."

But what I am doing for them now is just to feeling sorry.

"Just to feel sorry": implies that you are doing something so that you can feel sorry for them. I'm guessing that that's not quite what you meant, but I am not completely sure.

And I also feel guilty ofor having a dream tof becomeing an astronaut, notrather than helping those people.