Learn Turkish: Cidden çok yoruldum. - I am seriously very tired.

You could drop cidden çok and instead say Nasıl da yoruldum! (Oh, how tired I got!) or Ne kadar da yoruldum!. This gives the exact same intense feeling of exhaustion, but frames it as an exclamation rather than a standard statement.

The English translation says "I am tired" (present state), but yoruldum uses the past tense suffix -du. Why is that?

In Turkish, getting tired is treated as an event that has already happened, which results in your current physical state. Because the action of becoming tired is complete, you use the past tense yoruldum (literally: I got tired). This is the exact same logic used for acıktım (I am hungry / I got hungry).

How is the adverb cidden formed?

It is formed from the adjective ciddi (serious) using the suffix -en. This -en (or -an) ending is an Arabic suffix used in Turkish to create adverbs. You will frequently encounter it in other B2-level words like tamamen (completely), kısmen (partially), and şahsen (personally).

Can I use gerçekten instead of cidden in this sentence?