1. Take full responsibility for past performance, or lack thereof. It doesn't matter what you think could've, would've, should've with the instructional staff; it only matters what they think and did. Learn from that and don't complain, quibble, or make excuses. Just apply the lessons learned and move forward. Your opinion on the matter means nothing. You are merely a candidate. Think Richard Gere from An Officer and a Gentleman.
2. Ensure that for future apps and interviews, that you fully disclose all previous training successes as well as failures. Be up front first and foremost, and let the potential employer make the decision on whether that information is important to them or not. Do NOT fail to disclose something then have the employer find out later about it, to where what could've been a non-issue up front, now has them wondering "what else isn't being disclosed". Don't let a non-issue now become an issue.
3. When in training, ensure that you fully apply yourself to that training. When in training, barring any emergencies that must be handled, family life takes a backseat (assuming one isn't single). The home fires need to be kept burning and handled by those at home. If this can't be done while one is at training, there's no way in hell it can be done when you are really gone out on the line and on the road. That goes both for you the candidate, as well as family members. When TDY for training, ensure that the bulk of your off time is in the books. Don't burn yourself out, of course, but don't screw around or let other things become a priority if they don't absolutely have to be, most especially if you find yourself behind or having any kinds of trouble with the subject matter material. Take responsibility for your own training and progress, and make things happen on your end, always being prepared and showing up to training sessions as-such. Do not give the impression to any instructional staff that you are "just along for the ride".
4. Take command of your own training. Remember: you do not graduate training; your gradebook does.
The above are some major errors I've see from guys who have suffered setbacks such as yours and some of the things that have tripped them up from being able to advance past that.
You have the answers to what you need to do. Apply them. And remember, as someone trying to break into 121 such as yourself, the airline you make fun of or criticize today, just might be the one you're trying to interview with tomorrow. Tread carefully, especially on public forums.
Proceed.