Union with Christ

You know the problem well:
you seek to obey God and live a life that is pleasing and holy before him, but you continually fall into sin.
Then you repent and confess your sin to God with the promise of new obedience, but then you fall again, and the cycle of sin and repentance wears you down.
'How could God forgive me for the same sin, over and over?' you ask.
Is it true?
Sin shall not have dominion over you?