It started with a feeling, - the kind of feeling I tend to ignore until it's too strong.

A dash would probably be better here as opposed to a comma.

As asking for help was not an option, I could do only one thing: to escape.I could only do one thing to escape as asking for help was not an option.

This sentence is in passive voice. In writing it is less proper to use a passive voice in English. Passive voice shifts the focus from the subject doing the action to the recipient of the action. Also, the colon is used incorrectly here. Colons are used chiefly to introduce a list, quotation, or explanation following an independent clause. While your explanation was “to escape” the “to” was problematic.

It sounds slightly more native in my opinion to say “I could only do one…” rather than do only. They both make sense

You could choose to either say “ : escape” (which becomes the explanation needed for the colon to make sense grammatically)
“I could only do one thing: escape.”
Or you could say
“I could only do one thing to escape.”

The first sentence focuses the reader on your action - the escape. The second focuses the reader on the fact that you only had one option. It’s up to you to decide which to emphasize.

You can also set the second part of the sentence off in parentheses which gives the reader additional information while not impacting the continuity of the narration. “I could do only one thing to escape (as asking for help was not an option).”

As I was going down the stairs, my senses started failing;. I could barely hear,. It was as if I was under the water, and my reality was reduced to diffused and colorful shapes.

Instead of a semi colon this needs to be a period. This sentence is too long (called a run-on sentence) and needs a full stop. The second sentence is also a complete thought and therefore should also have a full stop. Also, it is more grammatically correct to say “it was as if” which both keeps the tense in past and emphasizes the simile nature of the comparison between the heat and the being under water experience.

You can leave the comma in before the and (called an Oxford comma) or you can remove it. The sentence will be read with different inflections depending on whether or not the comma is there.

"“No, this place is not the safest to allow myself that."unsafe. If I were to faint here….” (Add whatever you wish to the ellipsis.)

Technically grammatically correct, however, it does not sound native.

I sat, and after a few deep breaths, the difussed shapes became nitid.

Wow - you taught me a word! 😂 First time in a long time that has happened. I have never before seen or heard of nitid. 👍🏻

As asking for help was not an option, I could do only one thing: to escape.

omitting 'to' sounds a bit better

As I was going down the stairs, my senses started failing; I could barely hear, as if I was under the water, and my reality was reduced to diffused and colorful shapes.

"Am I going to faint?," I thought.

"No, this place is noI can't let myself faint the safest to allow myself thatre, it's too dangerous."

I sat up, and after a few deep breaths, the diffussed shapes became nitid.

I did not know the word 'nitid'