Όταν έχω πολλή δουλειά, προσπαθώ να αντιμετωπίζω το άγχος με λίγη ξεκούραση.

Because this sentence describes a habitual or general situation: whenever / when I have a lot of work.

In Greek, the present tense is very common for this kind of repeated meaning:

• Όταν έχω πολλή δουλειά... = When/Whenever I have a lot of work...

It is not talking about one single event only; it is talking about what usually happens.

Because δουλειά is a feminine singular noun, so πολύς has to agree with it.

Here are the relevant forms:

• πολλός = masculine
• πολλή = feminine
• πολύ = neuter / adverb

So:

• πολλή δουλειά = a lot of work
• πολύ νερό = a lot of water
• δουλεύω πολύ = I work a lot

In this sentence, πολλή is an adjective-like quantity word matching δουλειά.

Because δουλειά here means work as an uncountable idea, much like English a lot of work.

So:

• πολλή δουλειά = a lot of work

If you used the plural, it would suggest separate jobs or tasks:

• πολλές δουλειές = many tasks / chores / jobs

In this sentence, the singular is the natural choice.

Because προσπαθώ normally introduces another verb with να.

So:

• προσπαθώ να... = I try to...

This is one of the most common Greek patterns:

• θέλω να φύγω = I want to leave
• μπορώ να έρθω = I can come
• προσπαθώ να αντιμετωπίζω = I try to deal with / manage

English often uses the infinitive to, but Modern Greek does not have an infinitive in this use, so it uses να + verb form instead.

This is about aspect, which is very important in Greek.

• να αντιμετωπίζω = imperfective aspect
• να αντιμετωπίσω = perfective aspect

In this sentence, να αντιμετωπίζω suggests something ongoing, repeated, or habitual:

• I try to manage stress as a regular strategy

If you said να αντιμετωπίσω, it would sound more like:

• I try to deal with it once / successfully / as a complete action

Since the whole sentence is about what someone generally does when work is heavy, the imperfective αντιμετωπίζω fits very well.

Greek uses the definite article much more often than English.

So Greek naturally says:

• το άγχος = stress

Even with abstract nouns, Greek often keeps the article where English may omit it. Here, το άγχος is the normal way to say stress as the thing being managed.

Literally, it means:

• with a little rest

But in natural English, the idea is often:

• by getting a little rest
• through a little rest

Here με shows the means or method:

• αντιμετωπίζω το άγχος με λίγη ξεκούραση
  = I deal with stress with/by means of a little rest

For the same reason as πολλή δουλειά: agreement.

ξεκούραση is a feminine singular noun, so the word for a little must also be feminine:

• λίγος = masculine
• λίγη = feminine
• λίγο = neuter / adverb

So:

• λίγη ξεκούραση = a little rest
• λίγο νερό = a little water
• ξεκουράζομαι λίγο = I rest a little

άγχος is pronounced roughly like ANG-hos.

The cluster γχ is not pronounced like English g + h. In standard Greek, it is usually something like:

• [ŋx] before voiceless sounds
• [ŋɣ] in other environments

For a learner, a good practical approximation is:

• ANG-hos or AN-khos

Also note the stress:

• Άγχος → stress on the first syllable

The given order is the most neutral and natural one:

• Όταν έχω πολλή δουλειά, προσπαθώ να αντιμετωπίζω το άγχος με λίγη ξεκούραση.

Greek word order is more flexible than English, so some changes are possible for emphasis. For example:

• Προσπαθώ να αντιμετωπίζω το άγχος με λίγη ξεκούραση όταν έχω πολλή δουλειά.

That still makes sense, but it sounds slightly different in focus. The original version is a very normal way to present the idea.

Both can work.

In this sentence, because the meaning is habitual, whenever is often the clearest translation:

• Whenever I have a lot of work, I try to manage stress with a little rest.

But English often uses when for habitual meaning too:

• When I have a lot of work, I try to manage stress with a little rest.

So όταν can correspond to either one, depending on context.