I think they're easier to commit that one realizes. For example, adultery. Jesus said if you look upon a woman with lust in your heart, that's adultery.
If we are honest, this happens on a frequency most of us don't want to admit. And if we say it doesn't now we're lying.
However, the concept of mortal and venial sins and their impact on severing God's grace, that is His gift of salvation, is not Biblical.
Without faith in Christ, all sin seperates us from God. Belief in Christ restores and maintains our relationship with God.
God can remove that (and any) temptation completely, if you're serious. I know whereof I speak. Stay close to Christ and He will truly free you from sin.
Unless your definition of lust is "gosh she's pretty", then you have other problems to fix first.