Would any child respond, "Thanks for doing that for me, but I didn't deserve that so I might just go a try to clean it myself too."
No! They would go with joy, secure in knowing that their parent loved them, and freely receiving the gift they had just been given.
Why do when then do this very thing to God. "Come freely into my presence. I've already made you righteous!"
"Thanks God, but I didn't deserve that, so I might just go and try to work at it myself first."
We don't wash our clothes when they are already clean. We don't scrub the dishes when they are already spotless.
And yet we try to clean our own lives - something we could never do perfectly anyway - when Gods already done it.
Or worse, we sit there, giving up, thinking our lives will always be hopelessly dirty - and don't notice that Gods already saved and redeemed us.
Would they child, the room cleaned for them, then stay home and not even bother going to the party?
You can say all you like that real life is more complicated than the simple analogy I'm making, that's its not that easy.
But I believe, at the core of it, it is. It's that simple. God has paid the price. It's already done.
Accept it, like a child. Accept the gift, the joy, the peace. Let God deal with the complicated parts of your life. Just take the Grace.