John Newton (vicar in London and writer of Amazing Grace) addressed this question of the lack of growth, inconsistencies and sins of Christians. Why doesn't God deliver us from more of them? He has some incredibly insightful answers for us:
Firstly, "... he teaches us more truly to know and feel the utter depravity and corruption of our whole nature, that we are indeed defiled in every part. His method of salvation is likewise hereby exceedingly endeared to us; we see that it is and must be wholly of grace; and that the Lord Jesus Christ, and his perfect righteousness, is and must be our all in all."
Secondly, "..by what believers feel in themselves they learn by degrees how to warn, pity and bear with others. A soft, patient and compassionate spirit, and a readiness and skill in comforting those who are cast down, is not perhaps attainable in any other way."
Conclusion: God is sovereign even over my sin, and uses it - amazingly! - for my good and spiritual growth!