For those who do acknowledge that all salvation is by grace alone and have trusted in Christ alone for a righteous standing before God, you must believe that God is for you in your sanctification. Just as God was committed to bring Israel into the promise land, as Moses is teaching in Deut. 9, he is committed to transforming you into his Son’s image. He’s not out to trick you or put obstacles in your way.
That being said, we must not be deceived as to why God is for us. It’s not our own inherit goodness as if we had any anyways; any righteousness we do have is a gift from God and ultimately the result of his work in our lives. He has promised to complete the work he has started in Christians (Php. 1:6) and he will be faithful to his word. That must be the only source of our confidence. When it is, it sounds like this: “Lord, I know you’re not going to give me victory over my sin because of anything good in me, but because you have promised in your word to sanctify your children. I plead with you to give me victory over this sin on the basis on your promises to those who are in Christ. Help me believe in the abundant power I’ve been granted through Christ’s life, death and resurrection on my behalf.”
That’s how we can make sure we’re not trusting in our goodness, but instead in God’s resources and God’s faithfulness to his word.