Much of the difficulty you are having in this is that you are too new in your faith to know How to defend yourself and what to say.
But don't let that, or her words, get you down. Becoming a Christian is all about being a warrior. Whether it's fighting internal battles, or the ones that come from outside us, we are rebelling against our own nature. And against the nature of those around us. You are going to get backlash from that.
A healthy thing to learn is the difference between guilt and conviction. Guilt is what satan puts on us, or maybe we put on ourselves, but allows for satan to manipulate. Guilt feels bad. It feels so bad it freezes you up. And instead of change, you focus on how bad you are. You have trouble letting go. It brings you down and causes you to feel worse and not take action. This is not of God.
Conviction, on the other hand, is God's Spirit showing us our errors, putting that regret within us, but then helping us to learn from it and grow and move past the bad feelings. Because God has forgiven us, so learn to forgive yourself as well. Gods standards are way higher than yours, but He is able to forgive you. So see, it, feel it, change it, move on from it and become better. Or at least let it motivate you to begin working towards becoming better.
Lastly, you Are going to screw up. Period. We all do. Take the greatest preacher you can imagine. Take any of the 12 Apostles, and you know what you'll find? People screwing up and struggling with sin. But, again, we are warriors, so we fight it. We may not end all of our mistakes while we're alive, but we can lessen them. And some maybe we will be able to overcome eventually.
So when your wife gets on your case, just know that yes, there's a good chance you will mess up again. But that you are no longer giving in to your mistakes, but fighting them and learning from them. THAT is what living out being a Christian is all about.
As far as your wife, just do your best. Over time she will see a real change. It may take weeks, months or years. But if you are committed to Christ, she will see it when her hearts ready. Nothing you can do to rush that.