Como no seas más realista, te vas a frustrar cuando las cosas no salgan perfectas.

Here Como no means if you don’t or unless in a warning-like sense.

So:

• Como no seas más realista... = If you aren’t more realistic...
• It does not mean as or since here.

This structure is very common in Spanish when someone is giving a warning, caution, or mild threat about what will happen next.

Because after Como no used as a warning, Spanish normally uses the present subjunctive.

• seas = subjunctive of ser
• eres = indicative of ser

So:

• Como no seas más realista... sounds natural for a warning.
• Como no eres más realista... would be wrong here.

This is one of those patterns you learn as a chunk:

• Como no + subjunctive, ...

Yes, you could say Si no eres más realista, te vas a frustrar...

That version is grammatical, but the tone is a bit different:

• Si no eres... = more neutral, plain condition
• Como no seas... = stronger, more like a warning or caution

So Como no seas... feels a bit more emphatic, especially in spoken Spanish.

In Spanish, the normal expression is ser realista.

That is because realista usually describes a person’s general way of thinking, attitude, or outlook, not a temporary state.

So Spanish says:

• ser realista = to be realistic

But estar realista is not the normal choice here.

Because the comparison is implicit.

Más realista means:

• more realistic than you are now
• more realistic than your current expectations

Spanish often leaves that second part unstated when it is obvious from context.

So the full idea is something like:

• Como no seas más realista de lo que eres ahora...

But Spanish does not need to say all of that.

It means you’re going to get frustrated or you’re going to become frustrated.

A few parts are working together here:

• vas a + infinitive = near future / very common way to talk about the future
• frustrarse = to get frustrated, to become frustrated
• te = reflexive pronoun for you

So:

• te vas a frustrar = you are going to get frustrated

In everyday speech, this is often more common than the simple future:

• te frustrarás

Both are correct.

Yes. Both are correct.

With ir a + infinitive, the pronoun can go:

• before the conjugated verb: te vas a frustrar
• attached to the infinitive: vas a frustrarte

These mean the same thing.

In this sentence, te vas a frustrar sounds very natural.

Because after cuando, Spanish uses the subjunctive when the action refers to the future or to something that has not happened yet.

Here the idea is:

• when things don’t turn out perfectly in the future

So Spanish uses:

• cuando las cosas no salgan perfectas

Compare:

• Cuando las cosas no salen perfectas, me enfado.
  = habitual/general situation → indicative

• Cuando las cosas no salgan perfectas, te vas a frustrar.
  = future situation → subjunctive

Here salir means to turn out, to come out, or to go in the sense of how something ends up.

So:

• que las cosas salgan bien = that things turn out well
• que las cosas salgan mal = that things turn out badly
• que las cosas no salgan perfectas = that things don’t turn out perfectly

This is a very common use of salir in Spanish.

Because perfectas agrees with las cosas.

• las cosas = feminine plural
• therefore the adjective must also be feminine plural
• perfectas

So:

• la cosa perfecta
• las cosas perfectas

Even though English just says perfect, Spanish adjectives usually have to agree in gender and number.

It is mainly a warning, though it can also sound like advice depending on tone.

The structure Como no + subjunctive often sounds like:

• Watch out, because this will happen if you don’t...

So this sentence feels a bit stronger than a neutral statement. It suggests:

• you need to adjust your expectations,
• otherwise frustration is the likely result.

In Spain, this sounds natural and conversational, especially in spoken advice or criticism.