If God has "begun" a good work in me and God doesn't change His mind and He doesn't lie, then He will "perform" this "good work" to completion, right? What good work? Salvation. Does this mean that I, being imperfect and prone to being human will not sin? No, I will sin. I have been saved by His Grace. This imperfect sinner was saved by His Grace. God is the one who calls us. God is the one who saves us. God is the one who will complete what He has started. What kind of a God would we serve if He would call us and save us and then walk away. You say, "I walked away"...then I would have to say, "Were you ever 'truly' Born Again?" Salvation isn't emotion, fire insurance or something that sounds good. We don't pursue God, He pursues us. We don't choose God, He chooses us. He calls us. For His purposes.
"And we know that all things work together for good to those who love God, to those who are the called according to His purpose. For whom He foreknew, He also predestined to be conformed to the image of His Son, that He might be the firstborn among many brethren. Moreover whom He predestined, these He also called; whom He called, these He also justified; and whom He justified, these He also glorified. What then shall we say to these things? If God is for us, who can be against us?" Romans 8:28-31