Mi ansiedad puede convertirse en calma si respiro hondo.

Both can be possible in Spanish, but mi ansiedad is more personal and specific: it means my anxiety.

• mi ansiedad = your own anxiety, specifically yours
• la ansiedad = anxiety in a more general sense, or anxiety already understood from context

In this sentence, mi makes it clear that the speaker is talking about their own emotional state.

Because poder is a modal verb, like can in English.

Structure:

• puede = can
• convertirse = to become / to turn into

So:

• puede convertirse = can become

This is very common in Spanish:

• puedo ir = I can go
• puede cambiar = it can change
• podemos hacerlo = we can do it

After poder, the next verb stays in the infinitive.

Because the usual pattern for to become / to turn into is convertirse en.

• convertir usually means to convert something
  • Convirtió el garaje en oficina. = He turned the garage into an office.
• convertirse means to become / to turn into
  • La ansiedad se convirtió en calma. = The anxiety turned into calm.

So in your sentence, mi ansiedad is the thing that changes by itself, which is why the reflexive form convertirse is used.

Here, se is part of the verb convertirse. It is not a separate word with a literal meaning like himself or itself in English.

You should learn it as a full expression:

• convertirse en algo = to become something / to turn into something

So:

• convertir = to convert, to transform something
• convertirse = to become, to turn into

Many Spanish verbs change meaning when they become reflexive, so it is best to memorise convertirse en as one unit.

Because the verb pattern is convertirse en + noun.

Examples:

• convertirse en un problema = to become a problem
• convertirse en hábito = to become a habit
• convertirse en calma = to turn into calm

The preposition en is required by the verb expression. English uses into, but Spanish uses en here.

Because calma is being used as an abstract noun, almost like a state or quality.

• en calma sounds natural here: into calm / into a state of calm
• en la calma would sound more like into the calmness or a specific calm already identified, which is less natural in this sentence

Spanish often omits the article with abstract nouns in this kind of structure.

Because after si meaning if, you normally use a conjugated verb, not an infinitive.

So:

• si respiro = if I breathe
• si respirar ❌ = incorrect here

You need a full clause after si:

• si estudio = if I study
• si duermo bien = if I sleep well
• si respiro hondo = if I breathe deeply

Because Spanish uses the present indicative after si for real or possible conditions.

• si respiro hondo = if I breathe deeply
• si descanso = if I rest
• si practico = if I practise

This is the normal pattern for a likely or real condition:

• Si + present, present/future/can/etc.

In your sentence:

• Si respiro hondo, mi ansiedad puede convertirse en calma.

Because Spanish usually leaves out subject pronouns when the verb ending already shows who the subject is.

• respiro already means I breathe
• so yo respiro is possible, but usually unnecessary

Spanish is a pro-drop language, meaning pronouns like yo, tú, él are often omitted unless needed for emphasis or contrast.

So:

• si respiro hondo = normal
• si yo respiro hondo = more emphatic, as if contrasting with someone else

In this expression, hondo means deep.

• respirar hondo = to breathe deeply / to take a deep breath

This is a very common fixed expression in Spanish.

Even though hondo is literally an adjective meaning deep, in this phrase it works naturally with the verb respirar.

Other examples:

• Respira hondo. = Breathe deeply.
• Respiró hondo antes de hablar. = He/She took a deep breath before speaking.

Yes, you could, and it would be understood.

• respiro hondo = very natural, common, idiomatic
• respiro profundamente = also correct, a bit more literal/formal sounding

In everyday Spanish, especially in advice about calming down, respirar hondo is the more usual choice.

They often overlap in meaning, but they are not always interchangeable.

For breathing:

• respirar hondo = very common and idiomatic
• respirar profundo = not the usual expression
• respirar profundamente = correct, but less idiomatic than hondo

So for this sentence, hondo is the best choice.

Because si without an accent means if.

Compare:

• si = if
• sí = yes

Examples:

• Si respiro hondo... = If I breathe deeply...
• Sí, claro. = Yes, of course.

This is a very common distinction in Spanish.

A simple pronunciation guide:

• puede ≈ PWEH-deh
• hondo ≈ ON-doh

A few useful notes:

• In puede, the ue forms a diphthong, so it is one smooth syllable: pue-de
• The h in Spanish is silent, so hondo is pronounced as if it were ondo
• In Spain, the d between vowels is often softer than an English d

Yes. That is also correct.

You can place se:

• attached to the infinitive: puede convertirse
• before the conjugated verb: se puede convertir

So both are correct:

• Mi ansiedad puede convertirse en calma...
• Mi ansiedad se puede convertir en calma...

The first version is slightly neater and very common, but both sound natural.

Yes, it sounds natural and correct in Spanish from Spain.

Especially natural parts are:

• puede convertirse en
• respirar hondo

If anything, respirar hondo is a very typical, natural way to express breathe deeply in everyday Spanish.