Christians: help I feel so ashamed?

You don't mention what the sin are that you keep on doing--maybe it's not even a sin, but just a bad habit. But lets say it is a sin, then you have to really repent from your whole of sinfull way of life, and thinking, and convert unto God and His Son, and go as they say, and by doing that, you will crucify that which in actual fact is eating you alive from the inside.