We do believe that mortal sin, even as a believer, separates a Christian from the kingdom of Heaven because no one unclean can see God. Therefore, it is important to keep God's commandments and seek forgiveness when we sin.
No knowledgeable Catholic believes that God "owes" us anything for our labor. But "faith without works is dead", meaning that if you have faith but do not show your faith through your actions, your faith is meaningless or non-existent, because if you had faith in the first place you would automatically, through faith, try your hardest to live your life in the manner commanded by our Lord.
God bless.