"for it is God who worketh in you both to will and to work, for his good pleasure." (ASV)
"I've tried and tried to do it, but I just can't." So many people make this complaint about giving up old sinful habits. I'm here to tell you that they're right. No human can overcome sin. We aren't built to be able to. From the moment Adam and Eve bit into that apple way back in the garden, we're a fallen race. We can't escape sin on our own because it is a part of us.
There's good news though. We don't have to. The reason the above complaint is made is evident within the complaint itself. It's the emphasis on the word "I" that does it. If you're struggling against sin, stop struggling and be still. "I can do all things in him that strengtheneth me." (Philippians 4:13 ASV) All of your efforts may get you nowhere, but simply praying that God will take away the sin will have an enormous effect.
Just as a lump of ore can't refine itself, we humans need the refiner's fire to bring out our true potential. This is not an easy process to go through. "Behold, I have refined thee, but not as silver; I have chosen thee in the furnace of affliction." (Isaiah 48:10 ASV) Just as Israel endured difficulty and hardship in the Bible, so we too are refined today. When you ask for God's help in removing a sin from your life, be prepared. Often our faith is tested most when God is working hardest on us.
You see, it's not us, but God's work in us that perfects us. Where we are fallen, He alone is holy. Where we are weak, His strength will prevail. God knows the sins we struggle with, better than we do most times. He wants us to be made holy, "Ye therefore shall be perfect, as your heavenly Father is perfect." (Matthew 5:48 ASV) While the process will not be complete until we stand in His presence in heaven, He is working toward that end even today.
So the next time you find yourself wondering why you can't do something, remember that it's not yours alone to do. Give it to God in prayer and let Him handle it.