God's grace is available to help Christians to avoid sin.
But it depends on the individual circumstance. Sometimes the pull of sin is very strong and is not so easily overcome, such as in the case of dealing with addictions. Sometimes people simply love the sin too much and incline towards it and do not struggle against it enough, or accept the grace God offers. Sometimes our habits may simply direct us without our thinking, and we fall into sin almost accidentally, without even contemplating the temptation first. Speaking of temptation, VERY OFTEN it is just that - we are tempted, and instead of quickly rejecting the temptation at an early stage while it's pull on us is weak, we begin to consider it, and even experience a bit of pleasure at it (which can be the beginning of sin, even if we manage to reject the act itself in the end). But having considered and already enjoyed the temptation, we begin to reject any grace from God and decrease our desire to even resist.
There are several reasons we sin.
It is impossible to simply never, ever sin, in any thought, word, action, or attitude. We SHOULD sin less, and less, and less as we grow in the grace of God and become more like Christ, but even very sanctified people can expect not to be absolutely perfected in this life.
What do we do?
We accept our imperfection with humility. We accept responsibility for our sin when we fall. We ask forgiveness. We get back up. We turn from our sin. We continue to struggle against sin, with help from the grace of God. The next time we fall short and become aware of it, we do the same.
With effort, by the grace of God, with growth in Christ, we should sin lesss. But we are not sinless. It is our place in humility to accept this, and realize that we depend on the grace of God, and live by His mercy and depend upon that mercy as well, and ask God for grace and mercy, all of our lives.
If you want to hear more about how we fall into sin, and some tools to avoid it, this podcast is especially good. There is a transcript so you can read it, but listening is best for this one.
The Six Stages of Temptation - Acquiring the Mind of the Church | Ancient Faith Ministries