Without a sincere and diligent effort in every area of obedience, there will be no successful mortification of any one besetting sin. We may feel that a particular habit "isn't too bad," but continually giving in to that habit weakens our wills against the onslaughts of temptation from other directions. This is the reason, for example, that it is so important for us to develop habits of self-control over our physical appetites. We may think indulging these appetites isn't so bad, but such indulgences weaken our wills in every other respect of our lives.
Maybe the reason you can't overcome that big stronghold in your life is because you're not practicing holiness in the smaller things? Something to think about.