Learn Hardcore Portuguese (Portugal): Quando estou desmotivado, sinto uma grande frustração com qualquer pequeno erro. - When I am demotivated, I feel great frustration with any small mistake.

Quando estou desmotivado, sinto…
→ This describes a general / habitual situation: Whenever I’m unmotivated, I feel…
The present tense (estou / sinto) is used for repeated, typical behaviour.
Quando estiver desmotivado, vou falar com o meu chefe.
→ This describes a specific future situation: When I’m (one day) unmotivated, I will talk to my boss.
Here you use the future subjunctive (estiver) because it refers to a future event that may or may not happen.

In your sentence, the speaker is talking about a recurring pattern in their life, not one future event, so quando estou in the present indicative is the natural choice.

Why is it estou desmotivado and not sou desmotivado?

In Portuguese:

estar → temporary, changeable states or conditions (how you feel now or in certain periods)
ser → permanent or defining characteristics

Being motivated or unmotivated is usually seen as a state that can change, not as an inherent personality trait. So:

Estou desmotivado. → I’m feeling unmotivated (at this time / in this period).
Sou desmotivado. → Sounds like I’m (by nature) an unmotivated person – unusual and quite negative, describing your character, not just your mood.

That’s why estou desmotivado is the natural choice here.

Does desmotivado change if the speaker is a woman or if we talk about more than one person?

Yes, desmotivado agrees in gender and number with the person it refers to:

1 male speaker: estou desmotivado
1 female speaker: estou desmotivada
Group of only men / mixed group: estamos desmotivados
Group of only women: estamos desmotivadas

Same with third person:

Ele está desmotivado.
Ela está desmotivada.
Eles estão desmotivados.
Elas estão desmotivadas.

Could I say quando me sinto desmotivado instead of quando estou desmotivado? Is there a difference?

Yes, you can, and the meaning is very close.

Quando estou desmotivado
Literally: When I am unmotivated.
Focuses on your state / condition.
Quando me sinto desmotivado
Literally: When I feel unmotivated.
Focuses slightly more on your subjective feeling.

In practice, in this sentence, both are natural and almost interchangeable. Quando estou desmotivado is a bit more neutral and maybe slightly more common.

What’s the nuance of desmotivado? Could I also say desanimado or sem motivação?

All are possible, but they don’t feel exactly the same:

desmotivado
→ Lacking motivation, drive, or incentive. Often used with work, study, projects.
Very close to English unmotivated.
desanimado
→ Literally without spirit / without enthusiasm.
Feels more emotionally down or discouraged, not just lacking motivation. Closer to disheartened or down.
sem motivação
→ Literally without motivation.
Correct and clear, but a bit more formal / neutral, like something you’d see in a report or analysis.

In your sentence, desmotivado is the most natural everyday choice; desanimado would also work but shifts the focus slightly more to emotional discouragement.

Why is it sinto uma grande frustração and not something like sinto-me muito frustrado?

They are both grammatically correct, but they are different structures:

sinto uma grande frustração
- Verb sentir
  - noun (frustração)
- Literally: I feel a great frustration.
- Slightly more abstract / analytical; you are naming the emotion as a thing.
sinto-me muito frustrado
- Reflexive sentir-se
  - adjective (frustrado)
- Literally: I feel very frustrated.
- Sounds more personal and immediate, describing how you feel as a person.

In everyday speech, sinto-me muito frustrado is very common.
In your sentence, sinto uma grande frustração works well because it parallels qualquer pequeno erro – it presents frustration and errors in a slightly more conceptual way.

Why do we say uma grande frustração? Could I also say muita frustração or just grande frustração without uma?

All of these are possible, but with different nuances:

uma grande frustração
→ a great/big frustration (one strong feeling of frustration, or frustration of high intensity)
The article uma makes it feel like a single, big emotion.
muita frustração
→ a lot of frustration / much frustration
Focuses on the quantity/amount of frustration, not so much on it being one big “block”.
grande frustração (without *uma)
→ Grammatically possible, but you’d usually see this *after a preposition or in more literary contexts, e.g.
com grande frustração – with great frustration.
At the start of a noun phrase like in your sentence, uma grande frustração sounds much more natural.

So uma grande frustração is a very typical way to express a strong feeling of frustration.

Why is it com qualquer pequeno erro and not por qualquer pequeno erro?

Both com and por can appear in similar contexts, but they highlight different relationships:

com qualquer pequeno erro
Literally: with any small mistake.
Here com means in relation to / in the presence of / when faced with.
It suggests: As soon as a small mistake is involved, I feel frustration.
por qualquer pequeno erro
Literally: because of any small mistake / over any small mistake.
por focuses on cause. This version would also be understandable and not wrong, but it sounds a bit more like:
I get frustrated over any little mistake (that causes my frustration).

In European Portuguese, in this kind of emotional-reaction context, com is very natural:
Fico irritado com qualquer pequeno erro.
You could also hear por qualquer pequeno erro, but com is a safe, idiomatic choice here.

Why is it qualquer pequeno erro and not qualquer erro pequeno or pequeno qualquer erro?

Two things are going on: the position of qualquer and the position of adjectives in Portuguese.

Position of qualquer
- qualquer normally comes before the noun:
  qualquer erro – any mistake
- It does not go after the noun in this meaning, so erro qualquer is rare and has a different, more dismissive nuance (some random error).
So pequeno qualquer erro is incorrect.
Position of pequeno (adjective)
In Portuguese, many adjectives can go before or after the noun, but the meaning or nuance can change slightly.
- qualquer pequeno erro
  → Neutral, very natural: any small mistake (small in size/importance).
- qualquer erro pequeno
  → Grammatically possible, but sounds less idiomatic and a bit heavier; native speakers strongly prefer qualquer pequeno erro.

So the normal, natural order is:

qualquer pequeno erro = any small mistake

Why is there no article before qualquer? Why not com o qualquer pequeno erro?

In Portuguese, qualquer (meaning any) is a determiner that replaces the article; you normally don’t combine it with o / a / os / as.

So you say:

qualquer erro – any mistake
qualquer pequeno erro – any small mistake

but not:

✗ o qualquer erro
✗ o qualquer pequeno erro

The same rule applies in other contexts:

Qualquer pessoa pode aprender. – Any person can learn.
Em qualquer situação. – In any situation.

Could I add eu and say Quando eu estou desmotivado, eu sinto…? Is that wrong?

It’s not wrong, but it’s usually unnecessary and can sound heavy in this particular sentence.

Portuguese is a pro-drop language: the verb endings already show the subject, so you often omit subject pronouns:

Quando estou desmotivado, sinto… → natural, fluent
Quando eu estou desmotivado, eu sinto… → grammatical, but sounds a bit emphatic or repetitive in neutral speech.

You would normally add eu only for:

contrast:
Quando eu estou desmotivado, ela está cheia de energia.
emphasis: really stressing I.

In your neutral sentence, leaving eu out is the most natural choice.

Is the comma after quando estou desmotivado mandatory?

When a subordinate clause like quando estou desmotivado comes before the main clause, Portuguese writing conventions normally call for a comma:

Quando estou desmotivado, sinto uma grande frustração…

If you invert the order, the comma is usually dropped:

Sinto uma grande frustração quando estou desmotivado.

So in your original word order (subordinate clause first), keeping the comma is standard and recommended.