Learn Hardcore Indonesian: Ketika saya mulai pesimis, saya sengaja mengalihkan pikiran ke hal-hal kecil yang sudah berhasil saya capai. - When I start to feel pessimistic, I deliberately shift my thoughts to small things I have already achieved.

ketika = when (neutral, slightly written/formal, but very common)
saat = when / at the time (a bit more formal or “neat”; very common in writing and speeches)
waktu = when (literally time; feels a bit more casual in this kind of clause)

You could say, with essentially the same meaning:

Ketika saya mulai pesimis, …
Saat saya mulai pesimis, …
Waktu saya mulai pesimis, …

All are acceptable; ketika and saat sound a bit more neutral/standard than waktu in this kind of sentence.

Can I move the ketika-clause to the end of the sentence?

Yes. Both word orders are correct:

Ketika saya mulai pesimis, saya sengaja mengalihkan pikiran ke hal-hal kecil yang sudah berhasil saya capai.
Saya sengaja mengalihkan pikiran ke hal-hal kecil yang sudah berhasil saya capai ketika saya mulai pesimis.

The meaning is the same (When I start to feel pessimistic, I deliberately shift my thoughts…).
Putting ketika… at the beginning slightly emphasizes the time/situation. Putting it at the end sounds a bit more casual and less “written”.

Why is it mulai pesimis and not mulai merasa pesimis? Is it okay to drop merasa?

Yes, dropping merasa (to feel) is very natural in Indonesian.

mulai pesimis = literally start (to be) pessimistic
mulai merasa pesimis = start to feel pessimistic

Both are grammatically fine. Mulai pesimis is shorter and very common in speech and writing. Indonesians often use adjectives directly after mulai, for example:

mulai capek (start getting tired)
mulai marah (start getting angry)
mulai lapar (start getting hungry)

So you don’t need merasa unless you really want to emphasize the feeling aspect; even then, most people would still just say mulai pesimis.

What exactly does sengaja mean here, and where can I put it in the sentence?

Sengaja means deliberately / on purpose / intentionally.

In the sentence, saya sengaja mengalihkan pikiran… means I deliberately shift my thoughts… (not accidentally, but as a conscious strategy).

Typical placements:

Saya sengaja mengalihkan pikiran… (most natural)
Saya mengalihkan pikiran saya sengaja ke… (possible but feels a bit awkward here)
Dengan sengaja, saya mengalihkan pikiran ke… (more “written” or emphatic)

Using dengan sengaja instead of just sengaja is also correct but slightly more formal and heavier:

Saya dengan sengaja mengalihkan pikiran ke…

Most of the time, speakers just say saya sengaja ….

What is the literal meaning and structure of mengalihkan pikiran?

The base verb is alih, which has the idea of to move/shift/transfer (something to another place or focus).

mengalihkan = to shift / to divert (active verb with meN- prefix)
pikiran = thought(s)

So:

mengalihkan pikiran = to shift (one’s) thoughts / to divert (one’s) thoughts

Other common collocations:

mengalihkan perhatian = to divert attention
mengalihkan topik = to change/divert the topic

You can also add a possessive:

mengalihkan pikiranku = divert my thoughts
mengalihkan pikiran saya = same meaning, more neutral form

Why is it ke hal-hal kecil? What does hal-hal mean, and what is the function of the repetition?

Hal means thing / matter / aspect (abstract-ish “thing”).

In Indonesian, repeating a noun often makes it plural or “various kinds”:

hal = a thing / matter
hal-hal = things / various things

So:

hal kecil = a small thing
hal-hal kecil = small things

In this sentence, hal-hal kecil emphasizes multiple small achievements, not just one:

…mengalihkan pikiran ke hal-hal kecil yang sudah berhasil saya capai.
→ …shift my thoughts to the small things I’ve already achieved.

What does yang do in hal-hal kecil yang sudah berhasil saya capai?

Yang introduces a relative clause, similar to that/which/who in English.

Structure:

hal-hal kecil = small things
yang sudah berhasil saya capai = that I have already managed to achieve

So:

hal-hal kecil [yang sudah berhasil saya capai]
= the small things [that I have already succeeded in achieving]

Yang links the noun hal-hal kecil with extra information about it, just like:

the things that I achieved
the person who helped me
the book which I bought

Why does the clause say yang sudah berhasil saya capai and not yang sudah berhasil saya mencapai?

In Indonesian, when the object is being “relativized” with yang or moved in front of the verb, the verb often appears in its base form (without meN-), and the subject pronoun comes before the verb:

Saya sudah mencapai hal-hal kecil itu.
(I have already achieved those small things.)

When hal-hal kecil itu becomes the focus with yang, the pattern changes:

Hal-hal kecil itu sudah saya capai.
Hal-hal kecil yang sudah saya capai.

So in the relative clause, capai (base form) is used, not mencapai.
Yang sudah berhasil saya capai follows this pattern:

(Hal-hal kecil) yang sudah berhasil saya capai.

…saya mencapai after yang is ungrammatical in this structure. You either say:

yang sudah saya capai, or
yang sudah berhasil saya capai (adds the idea of “successfully”).

What is the nuance of using both sudah and berhasil together? Aren’t they redundant?

They are not redundant; they express different ideas:

sudah = already (aspect: completed)
berhasil = succeeded (success, achievement)

So:

yang sudah saya capai = things that I have already reached/achieved
yang berhasil saya capai = things that I managed to achieve (successfully), maybe after some effort
yang sudah berhasil saya capai combines both:
- already done
- and achieved successfully

You could grammatically say:

hal-hal kecil yang sudah saya capai (perfectly fine)
hal-hal kecil yang berhasil saya capai (perfectly fine)

Adding both sudah and berhasil gives a slightly stronger sense of things I’ve already successfully accomplished.

Could I say yang sudah saya capai instead of yang sudah berhasil saya capai? Does it change the feeling?

Yes, you can say:

…hal-hal kecil yang sudah saya capai.

It is fully correct and natural.
The nuance:

yang sudah saya capai = neutral: that I have already achieved
yang sudah berhasil saya capai = adds a subtle sense that these were goals that required effort and that you succeeded in them

So berhasil adds a bit of emotional weight or emphasis on success, but it’s optional.

Is this sentence more formal, informal, or neutral? Could I say it in everyday conversation?

The sentence is neutral Indonesian:

All words are standard and common.
The structure is natural for both spoken and written language.

You could absolutely say this in normal conversation, for example when talking about how you manage negative thoughts. In very casual speech people might shorten or simplify it a bit, but as-is, it sounds natural and not overly formal:

Ketika saya mulai pesimis, saya sengaja mengalihkan pikiran ke hal-hal kecil yang sudah berhasil saya capai.