Learn Hardcore Spanish (Spain): Hoy estoy cansado; aun así, quiero seguir estudiando español. - Today I am tired; even so, I want to keep studying Spanish.

Estoy cansado = I am tired (right now / today; it’s a temporary condition).
Soy cansado would suggest that being tired is an essential or habitual characteristic of you (almost like “I’m a tiring person” or “I’m someone who’s always tired”), and it sounds strange in most contexts.

So with feelings and physical states that change, like cansado, contento, triste, enfermo, you normally use estar.

Why is it cansado and not cansada? What if a woman is speaking?

Adjectives must agree in gender and number with the subject:

In your sentence, cansado assumes the speaker is male (or that we’re using the masculine as a default). A female speaker would say:

Why is there a semicolon (;) instead of a comma or a word like pero?