Not carrying the tune

One of the Pastors use this anectdote quite a few times in his sermons. It is by Mark Twain
Mark Twain's wife was not enamored of her husband's cruder domestic compositions. One morning, having cut himself while shaving, Twain cursed to high heaven and beyond.
When the barrage finally stopped, Twain's wife, endeavoring to shame him into an apology, repeated as many of her husband's profanities as she could recall.
"You have the words, my dear," Twain nonchalantly replied, "but I'm afraid you'll never master the tune

I think this is what describes me and my christian life. as much as I try to rattle in christianity. It doesn't work. It just doesn't seem to fit me.

I just so feel like a shizophrenic Christian. It is up and down up and down. never an even keel.The truth isI wonder "Can I ever truly change from what I came from?"
I know God's grace can change me, but if it can change every aspect of a person's life.Why must we look at someone's family when we consider marriage? Why does that play in consideration? What does that mean?
In Institute Preacher talked of marriage. He taught that we should look at the person's family. Because we emulate our family. If that is the case, then there was never any hope for me to change. There was never any chance of me meeting a good Christian man, thus me marrying a nonChristian that was good should be acceptable.

I am not trying to make excuses. I am just wondering were God's grace fit in to this rationale. You can't compare apples to oranges. You can't compare the saved with the unsaved and say they are the same. The difference is God's grace right?

maybe I can't carry the tune of Christianityvery well, but I also can't carry that of a nonChristian life either.