Learn Hardcore Russian: Иногда мне не хватает мотивации в будни.

You cannot say я не хватает, because хватает is 3rd person singular; it doesn’t agree with я. The verb is not describing what I do, but a state that applies to me.

Я не имею мотивации is grammatically possible but sounds very formal, legalistic or unnatural in everyday speech. Native speakers almost never say it in this context. They use:

Мне не хватает мотивации.
У меня нет мотивации.

What exactly does мне не хватает mean, and how is it used?

Хватать / хватить means “to be enough / to suffice.”

The pattern here is:

[Dative person] + (не) хватает + [Genitive thing]

Examples:

Мне хватает времени. – I have enough time.
Мне не хватает времени. – I don’t have enough time.
Ей хватает денег. – She has enough money.
Им не хватает опыта. – They don’t have enough experience.

So Мне не хватает мотивации = “I don’t have enough motivation.”
The negative just adds не before хватает.

Why is мотивации in the form мотивации, not мотивация?

Here мотивации is in the genitive singular, governed by (не) хватает.

With хватать / не хватать, the thing that is (not) enough takes the genitive case:

Мне хватает мотивации.
Мне не хватает мотивации.

Other examples:

Мне не хватает терпения. – I don’t have enough patience.
Нам хватает сил. – We have enough strength.

So we use genitive because the idea is “enough / not enough of motivation,” not “motivation” as a subject in the nominative.

Could мотивации here be plural (“motivations”), or is it singular?

In this sentence it is genitive singular, meaning “motivation” in general, treated as a mass/abstract noun.

Forms of мотивация:

Nominative singular: мотивация
Genitive singular: мотивации
Nominative plural: мотивации
Genitive plural: мотиваций

So the form мотивации is ambiguous in isolation, but:

Хватает / не хватает requires genitive, not nominative.
The meaning “motivation” as a personal drive is normally uncountable.

Plural мотивации (nominative plural) is used in a different sense, like “motivations / motives”:

Его мотивации были непонятны. – His motivations were unclear.

What’s the difference between Мне не хватает мотивации and У меня нет мотивации?

Both are natural, but the nuance is different:

Мне не хватает мотивации.
“I don’t have enough motivation.”
Implies you have some, but it’s insufficient.
У меня нет мотивации.
“I have no motivation.”
Stronger: you completely lack motivation.

In everyday speech:

Мне не хватает мотивации в будни. – sounds a bit softer, like “My motivation is low on weekdays.”
У меня нет мотивации в будни. – sounds more absolute and dramatic: “I have zero motivation on weekdays.”

What does в будни mean exactly, and can I say по будням or в будние дни instead?

В будни means “on weekdays / during weekdays.”

You can also say:

по будням – on weekdays (regularly)
в будние дни – literally “on weekday days”

All three are correct and close in meaning:

Иногда мне не хватает мотивации в будни.
Иногда мне не хватает мотивации по будням.
Иногда мне не хватает мотивации в будние дни.

Nuance:

по будням is very common for habitual actions.
в будни is also natural and slightly shorter.
в будние дни is a bit more explicit/longer, sometimes a bit more bookish or careful speech.

What case is будни in, and why doesn’t the form change?

Here будни is accusative plural.

The noun paradigm is:

Nominative plural: будни
Accusative plural: будни (same as nominative; it’s inanimate)

Preposition в + accusative is used for time expressions like days:

в понедельник – on Monday
в выходные – on the weekend(s)
в будни – on weekdays

So в будни = в + accusative plural.

Can I change the word order, for example: Мне иногда не хватает мотивации в будни?

Yes, Russian allows flexible word order. All of these are grammatically possible, with slight differences in emphasis:

Иногда мне не хватает мотивации в будни.
Neutral, fairly typical: “Sometimes, I don’t have enough motivation on weekdays.”
Мне иногда не хватает мотивации в будни.
Very natural. Slightly more focus on мне (“For me, sometimes there isn’t enough motivation on weekdays”).
В будни мне иногда не хватает мотивации.
Emphasizes the time frame: “On weekdays, I sometimes don’t have enough motivation.”

What sounds unnatural is putting иногда far from the verb, like:

Мне не хватает мотивации иногда в будни. – technically possible but clumsy; natives would avoid this order.

Why is не хватает in the present tense if I’m talking about a general, repeated situation?

Russian uses the present tense for:

actions/states happening now, and
regular / habitual actions in general.

So Иногда мне не хватает мотивации в будни =
“Sometimes I lack motivation on weekdays” – a general, habitual fact, so present is correct.

If you talk about a period in the past, you’d use the past:

Иногда мне не хватало мотивации в будни.
“Sometimes I lacked motivation on weekdays (back then / during that time).”

Perfective хватить is used mostly for future or completed situations:

Мне хватит мотивации. – I will have enough motivation.
Мне не хватило мотивации. – I didn’t have enough motivation (in that specific situation).

How do you pronounce this sentence, and where is the stress?

Stresses:

Иногда – иногда (stress on the last -да)
мне – one syllable, unstressed here
не – unstressed
хватает – хвотает (stress on -та-)
мотивации – мотивации (stress on -ва-)
в – unstressed
будни – будни (stress on бу-)

Approximate pronunciation (with stressed syllables in caps):
ee-na-gdA mnye ne khva-TÁ-yet ma-tee-VÁ-tsee-ee v BÚD-nee

Note the soft дн cluster in будни: the д is slightly softened before н. The о in иногда is reduced and sounds more like a, as in most unstressed о in Russian.