What should I do and how can I fix what has passed me by, now that I have the full intention to repent, after having strayed from the religion and committed great sins?

Repentance from major sins is obligatory. Praise be to Allah that you have felt His mercy and desired to repent, for Allah forgives all sins for whoever repents and turns to Him. Allah says: (And My mercy encompasses all things). And He says in a Hadith Qudsi: "O son of Adam, were your sins to reach the clouds of the sky, and then you sought My forgiveness, I would forgive you, and I would not mind."

The conditions for sincere repentance are:

1. Abandoning the sin and cutting off the unlawful relationship. 2. Regretting what has passed. 3. Determining not to return to the sin again.

Increase in good deeds and effacing merits, for Allah Almighty says: (Indeed, good deeds do away with evil deeds). And seek out righteous companionship that will help you in obedience to Allah.