I suspect that if a poll were taken of all the people who made a list of New Year's resolutions this year, the majority would probably say what you've said. Jesus' words apply to us all: "The spirit is willing, but the body is weak" (Mark 14:38).
Why can't we keep our resolutions to become better people? Sometimes, of course, it's because our resolutions haven't really been resolutions at all, but only been vague wishes. Other resolutions may have been sincere, but they were unrealistic, or the person who made them had no practical plan to reach their goal. I might resolve to lose 30 pounds, for example, but if I have no realistic plan to do so, I probably won't get it done.
But the real problem is inside of us -- within our own hearts and minds. Although we may hate to admit it, we are morally and spiritually crippled, unable to do everything that's right and turn away from everything wrong. The reason, the Bible says, is because we all have a spiritual "disease" -- the "disease" of sin.
This is why we need Christ, for only He can forgive our past and give us the strength we need to do what is right. Don't despair, but repent of your sins and by faith ask Christ to come into your life. Then ask Him to help you become the person He wants you to be -- and He will, as His Spirit works within you. The Bible says, "I can do everything through him who gives me strength" (Philippians 4:13).