Learn Spanish (Latin America): No es que la timidez me controle, sino que me gustaría estar solo. - It is not that shyness controls me, but rather that I would like to be alone.

This sentence uses the structure no es que + [rejected reason] + sino que + [actual reason]. The rejected reason (la timidez me controle) takes the subjunctive because you are denying that it is the cause. The actual reason (me gustaría estar solo) uses the conditional (which acts like the indicative here) because you are declaring it as the real truth.

Why do we use sino que here instead of just sino?

You use sino to mean 'but rather' when correcting a negative statement. However, you must add que when the following part is a full clause with a conjugated verb. Since me gustaría is a conjugated verb, you need sino que. If you were just contrasting nouns or adjectives, you would only use sino.

Why is there a la before timidez when the English translation is just 'shyness'?

In Spanish, abstract nouns like timidez (shyness), amor (love), or paz (peace) require the definite article (el or la) when they are the subject of the sentence or discussed in a general sense.

Could I use ser instead of estar in estar solo?