When you are miserable enough — when you are sick and tired of being sick and tired — a switch gets flipped inside, and nothing else matters except the will to get out of the hole you are in. I can only describe it as a survival instinct.
The over-simplified answer is this. You have to start by creating a vision of who you want to be, because clarity is power. Focus on it. Meditate on it, every single day. Know that everything truly worthwhile in life requires a leap of faith.
If you do not feel worthy of the things you love, nothing you learn or do will ever stick. Self-esteem and self-worth are the foundation upon which a person's identity is built, and for most, repairing the damage done by the past is a huge hurdle to overcome. It was for me.
A mindset of devotion is required, no matter the failures or setbacks. Habits harden over time regardless of failure, and the process will always include failure. Failure is only relevant in how fast you can recover from it. You cannot let it become an excuse to quit. When it happens, let it go.
Forgive yourself and release the guilt of your past. In my mind, every mile I ran was a form of atonement. Evict everyone and everything that limits your ability to evolve, and surround yourself with people who motivate you to be the best version of yourself. Who you surround yourself with is ultimately who you become.
Know that nothing about this process is easy, and that you will need to sacrifice everything you are for who you want to become.