I've known her since we were children, but we only became friends only in middle school.

“Only” sounds better in this part of the sentence

BecausSince I was still recovering from pneumonia, I didn't visit my hometown when I was on vacation since I worried that the long journey could harm my health.

“Since” sounds better than “because” here

I'm drowning in routine and feel isolated.

This sounds a little odd, but I think it technically works

I don’t know how to explain this feeling:, it's as if nothing else exists, never has existed, and never could exist.

A colon should not be used here, you should use a comma instead

Anyway, I'm looking forward to our meeting.

Alternatively you could use the word “reunion” here since I’m assuming you two haven’t seen each other in a while

I've known her since we were childrenvery young, but we only became friends only in middle school.

I think most adults would consider middle school students still children. In the United States, middle school is about ages 11-14. I would say "very young" or "little children" or something to emphasize even younger than middle school.

Because I wasam still recovering from pneumonia, I didn'tecided not to visit my hometown when I was on vacation since I. I was worried that the long journey could harm my health.

Since this is happening now/in the future, I would change the tense.

You know, I feel like a hamster on a wheel.

This is a big change in subject. I'd create a new paragraph, and also look for some way to connect your friend's visit with how you are feeling. Something like "It will be especially nice since I've been going through a rough time. I feel like a hamster on a wheel."

I'm drowning in routine and feel isolated.

AIt's as if a previous version of me died or never existed at all, and my past is just a dream, and the future will never come.

I know she's still my best friend, but... it's like she's the best friend for the current version of me.

I seemtend to forget how long our story is, and I can't believe all this happened to us, not to someone elsehow much we have been through together.

It's not clear from your story what happened to you. If you want to keep it generic, maybe say something like I've suggested above.