The Lord endured continous temptations while He was in the world. If the regeneration process were instantaneous, we'd be perfect, which is not the case for anyone but the Lord.
Regeneration simply means we have a spirit that is living as God has regenerated that spirit in us. Our previous spirit was dead from God (some might argue for dichotomous anthropology sating unbelievers simply have no spirit,..wrt a proper fellowship with God through His plan, both interpretations are identical for we need not dwell on how the dead remian dead, but focus instead upon a life through our Lord Christ Jesus, a Living God)(another definition of death is a state of exstence involving separation).
Our perfection in sinlessness falls under the doctrines or continuing faith of sanctification, not regeneration. Upon our initial saving faith in Christ, we are positionally sanctified with God and saved by His work. We still retain an old sin nature, and while remaining in fellowship with Him, may continue to advance in sanctification.
Regeneration only occurs once.
Eternal life begins at regeneration, when we believe in the salvation work of Jesus Christ, the Holy Spirit imputes life to the human spirit. His decision to give us life as been predestined and is a perfect decision by God the Father in eternoty past.
There is nothing we can do nor anything God would ever do to reject that eternal life once we have been regenerated. This doesn't mean we don't sin after salvation, nor does it mean we don't continue to be sanctified by the Holy Spirit as we remain in fellowship with Him and respond to all things through faith in Him, by His problem solving devices. This point of irrevocable eternal life is that God has already predestined our eternal life and His decisions are Perfect. If He were to allow any of those regenerated to lose eternal life, then there is no grounds for any to retain eternal life. Eternal life is not a question of morality. It is only based upon faith in Him and His work by grace.
You are quite correct to note we continue to resist temptation and are able to do so through faith in Him.