Cuando estoy ansioso, mi respiración se acelera.

In Spanish, estar is used for temporary states, conditions, or emotions, while ser is used for more permanent characteristics or identities.

• Estoy ansioso = I am feeling anxious (right now / in this situation).
• Soy ansioso = I am an anxious person (in general, by nature).

In your sentence, the anxiety is treated as a temporary emotional state, so estar is the correct verb:
Cuando estoy ansioso, mi respiración se acelera.

Ansioso/ansiosa must agree with the gender of the speaker:

• A man would say: Estoy ansioso.
• A woman would say: Estoy ansiosa.

So the full sentence changes depending on who’s speaking:

• Male speaker: Cuando estoy ansioso, mi respiración se acelera.
• Female speaker: Cuando estoy ansiosa, mi respiración se acelera.

Everything else in the sentence stays the same.

Both are grammatically possible, but there’s a nuance:

• Mi respiración = my breathing, stressing that it’s my specific breathing.
• La respiración = the breathing, a bit more general or impersonal (though context can still make it clear it’s yours).

In this sentence, Spanish speakers usually prefer mi respiración to clearly connect the physical reaction to the speaker’s own body.

Compare:

• Cuando estoy ansioso, mi respiración se acelera.
  When I’m anxious, my breathing speeds up (my own body’s reaction).

• Cuando estoy ansioso, se acelera la respiración.
  More like When I’m anxious, the breathing speeds up—stylistically okay, but a little less direct and personal.

Yes, acelerarse here is used as a pronominal (reflexive-like) verb.

• Acelerar = to speed something up (transitive)
  
  • El conductor acelera el coche. – The driver speeds up the car.
• Acelerarse = to speed up (in itself) / to become faster
  
  • Mi respiración se acelera. – My breathing speeds up.

The “se” shows that “la respiración” (the breathing) is undergoing the change itself, not being acted on by someone else. This is very common with physical or emotional changes:

• El corazón se acelera. – The heart speeds up.
• La situación se complica. – The situation gets complicated.
• La discusión se calienta. – The argument heats up.

You can say it, but you need to turn acelerar into a normal, non-pronominal verb. Two natural options:

1. Mi respiración acelera.
   Grammatically correct, but sounds a bit unusual or technical in everyday speech.

2. Algo acelera mi respiración.

   • La ansiedad acelera mi respiración. – Anxiety speeds up my breathing.
   • El estrés acelera mi respiración. – Stress speeds up my breathing.

In normal, everyday Spanish about bodily reactions, “mi respiración se acelera” is much more idiomatic than “mi respiración acelera.”

The present tense in Spanish, as in English, is often used to talk about:

• Habits or things that are generally true:
  • Cuando estoy ansioso, mi respiración se acelera.
    When I’m anxious, my breathing speeds up (this is generally what happens to me).

Other possible tenses, with different meanings:

• Past habitual:
  Cuando estaba ansioso, mi respiración se aceleraba.
  When I was anxious, my breathing would speed up / used to speed up.

• Future:
  Cuando esté ansioso, mi respiración se va a acelerar.
  When I’m anxious (in the future), my breathing is going to speed up.

The original uses present because it’s a general, timeless pattern.

Whether you use indicative or subjunctive after cuando depends on the meaning and time reference.

• Indicative (estoy) for things that are general, habitual, or considered real:

  • Cuando estoy ansioso, mi respiración se acelera.
    Whenever I am anxious, my breathing speeds up. (General rule)

• Subjunctive (esté) for future or hypothetical situations in many contexts:

  • Cuando esté ansioso, trataré de respirar profundo.
    When I’m anxious (at some future time), I’ll try to breathe deeply.

In your sentence, you’re talking about a general fact about yourself, so the indicative “estoy” is correct.

Yes. Both word orders are correct in Spanish:

• Cuando estoy ansioso, mi respiración se acelera.
• Mi respiración se acelera cuando estoy ansioso.

Two points:

1. Comma usage

   • If the cuando clause comes first: you do use a comma.
     Cuando estoy ansioso, mi respiración se acelera.
   • If it comes after the main clause: usually no comma.
     Mi respiración se acelera cuando estoy ansioso.

2. Meaning
   The meaning is the same; it’s mostly about rhythm and emphasis.

They’re close in meaning but not identical, and the preference can be regional or personal.

• Ansioso / ansiosa

  • Often implies anxiety, restlessness, worry, tension.
  • In some places, also “eager” or “excited to get something”:
    • Estoy ansioso por verte. – I’m eager to see you.

• Nervioso / nerviosa

  • More like nervous, jittery, on edge, agitated.
  • Very common in everyday speech for what English speakers might call “anxious”.

You could also say:

• Cuando estoy nervioso, mi respiración se acelera.

In many everyday contexts in Latin America, nervioso is more common for “anxious” feelings, but ansioso is perfectly correct and often a bit closer to “anxious” in the clinical/psychological sense.

Yes, and it’s very natural in spoken Latin American Spanish.

• Mi respiración se acelera.
  Neutral, straightforward: My breathing speeds up.

• Se me acelera la respiración.
  Very idiomatic way to talk about an involuntary bodily reaction. Literally something like: The breathing speeds up on me.

The “me” here marks that the reaction is happening to you, almost outside your control. It’s common with physical or emotional reactions:

• Se me acelera el corazón. – My heart races.
• Se me cierran los ojos. – My eyes are closing (on their own).

So you could say:

• Cuando estoy ansioso, se me acelera la respiración.
  Same meaning, slightly more colloquial and “felt” as an involuntary reaction.