Previous "Failures" are a good thing as long as you can explain how you improved as a person and as a crew member/pilot. Frontier, or any airline for that matter, wants to see how you will potentially deal with adversity during your career as one of their employees. You are definitely going to experience another "failure", be it a training issue or an airplane system issue while out on the line, or an interpersonal issue with the myriad of fellow crew members you interact with during your career.

Can you take accountability or is it always the other person's fault?
Can you work as a team member towards a resolution or do you fixate on "who" is right vs "what" is right?
You passed subsequent, more challenging oral tests after your CFI. How did you improve yourself and your preparation for future successes after previous failures?

In short, look at it as an opportunity to explain why you are a better person overall.