Why is "isn't" the current form here?
We know that "isn't" is a contraction of "is not." If we make the sentence more concise by first removing expressions like "sure as hell," we are left with "it isn't my fault that you didn't turn out to be an actress." Because the speaker is denying any responsibility in his wife not being able to achieve her dreams, he has used the current tense "isn't" here. "Didn't" is already in the past tense, but use of this expression requires its connected verb to be in the present infinitive form after (such as "sleep," "eat," "run"). "Turn out" is the infinitive form linked to "didn't," so thus we can see that it is only appropriate to use "isn't" here to make sure the tenses stay consistent. Ex: It was my fault that the trip got canceled. I didn't check the weather before planning. Ex: It's my fault that I didn't bring my cell phone. You reminded me before we left home.