Mam krótkowzroczność.

Polish often uses mieć (to have) for illnesses, conditions, and medical problems.

So:

• Mam krótkowzroczność = I have myopia / nearsightedness

This is completely correct.
Polish can also use an adjective:

• Jestem krótkowzroczny — if the speaker is male
• Jestem krótkowzroczna — if the speaker is female

That version is often more everyday and feels closer to English I’m nearsighted.

It is in the accusative, because mieć normally takes a direct object in the accusative:

• Mam co? → krótkowzroczność

The useful thing to notice is that this noun looks the same in the nominative and accusative, so there is no visible ending change here.

Because many feminine nouns of this type keep the same form in the nominative and accusative singular.

So:

• nominative: krótkowzroczność
• accusative: krótkowzroczność

Even though the case changes grammatically, the spelling stays the same.

No. In Polish, subject pronouns are often omitted because the verb already tells you the person.

• Mam krótkowzroczność = normal
• Ja mam krótkowzroczność = possible, but emphatic

You would usually add ja only for contrast or emphasis, for example:

• Ja mam krótkowzroczność, ale on nie ma.
  I have myopia, but he doesn’t.

No. Mam krótkowzroczność is the same for everyone.

That is because:

• mam does not show gender
• krótkowzroczność is just a noun, not an adjective agreeing with the speaker

But if you use the adjective version, then gender matters:

• Jestem krótkowzroczny — male speaker
• Jestem krótkowzroczna — female speaker

Yes, it is correct and natural, especially in a medical or explanatory context.

That said, in everyday speech some people may more often say:

• Jestem krótkowzroczny / krótkowzroczna
• Źle widzę z daleka — I don’t see well at a distance
• Mam wadę wzroku — I have a vision defect

So Mam krótkowzroczność is fine, but it can sound a little more clinical than the simplest everyday phrasing.

A rough English-friendly approximation is:

kroot-koh-VZROCH-noshch

A few helpful sound notes:

• ó sounds like u in Polish, so krót- sounds like kroot
• rz sounds roughly like the s in measure
• cz sounds like ch in chop
• ść at the end is a soft consonant cluster, roughly shch

The stress is on the second-to-last syllable, which here is wzrocz:

krót-ko-WZROCZ-ność

And the whole sentence is roughly:

mam kroot-koh-VZROCH-noshch

Yes. It is related to the idea of short-sightedness.

You can connect it with:

• krótki = short
• wzrok = sight / eyesight

Also, the ending -ość often makes an abstract noun, similar to English -ness.

So you can think of krótkowzroczność as something like:

• short-sighted-ness

That makes it easier to remember.

Because Polish does not use articles like English a / an / the.

So Polish simply says:

• Mam krótkowzroczność

not something like Mam a krótkowzroczność or Mam the krótkowzroczność.

The meaning is understood from context.

You say:

Nie mam krótkowzroczności.

Notice the change:

• positive: Mam krótkowzroczność
• negative: Nie mam krótkowzroczności

After a negated verb like nie mam, Polish normally uses the genitive, so the noun changes from krótkowzroczność to krótkowzroczności.

Yes, it can.

So depending on context:

• literal: eye-related short-sighted / myopic
• figurative: short-sighted, meaning lacking foresight

However, the noun krótkowzroczność in a sentence like Mam krótkowzroczność is understood literally as the medical eye condition.