One of the most beautiful doctrines in all of Christianity is the Perseverance of the Saints. Many of us fear the loss of salvation. I've sinned, so will God just send me to Hell anyway? But the truth is, because salvation is obtained by grace, through faith, the only way to "lose" salvation would be to fall away from grace by losing faith.
But because God is so gracious, it is impossible to truly lose faith. Of course, by that, I don't mean it's impossible for someone who believes in God to stop believing in God. We see a lot of people doing that (1 John 2:19 tells us that this is so it could be made clear that they were never truly of the faith). This is because there are two different types of faith. Those who leave the faith gave their intellectual assent to the existence of God, but as James tells us, even Satan believes in God. How does he respond? Not well, as it turns out.
But how do you respond? If your faith is true, going beyond mere intellectual assent, your faith will never fail. God preserves His saints. As Paul tells us, He who started a good work in you will carry it on to completion until the day of Christ Jesus.
This actually answers why we still fall into sin. It's not like we convert and then boom, we're instantly Jesus. Think of it like this: If a church building gets burned down, it can be rebuilt. But the mere decision to rebuild it doesn't make it instantly perfect. It takes time to rebuild, and while it is being rebuilt, there will be a lot of problems. In the same way, God is working on us. If you're not perfect yet, it's not because God has abandoned you, neither will it cause God to abandon us. It just means that God isn't finished with you yet. But fear not, for on the day of Jesus Christ, you will be finished.