There is no single thing. Anyone who tells you that has not done serious time or serious self-examination. What actually works is a combination of choices made consistently over time, and here is what experience says matters most.
Forgive yourself, and forgive the people who will not forgive you. You created this. Owning that without drowning in it is what allows you to actually move forward. Resentment toward others or toward yourself is a chain that keeps you connected to the past.
Look hard at who you spend time with. There is a phrase that holds up: show me your friends, and I will show you your future. The people from the old life, the ones connected to the behavior that cost you, are a dead end. That is not judgment; it is math. New life requires new people.
Set goals you can actually reach. Not dreams, goals. Concrete, realistic steps you can take this week, this month, this year. Focus on what you can control and let go of what you cannot.
Make peace with a normal life. A quiet night where you can lay your head down without waiting for the knock at the door is worth more than any hustle that kept you up at night. An honest living that is not lavish but is yours, fully and without fear, is the life worth building.
Be grateful for every day that is not behind bars. That gratitude, practiced daily, keeps the alternative in perspective.