Thats an easy one. You say "Yes, I was asked to resign from so-and-so airlines training program, which in hind sight was a good thing. I took that opportunity as a valuable learning experience that I kept with me while flight instructing and learning this past year an a half. I have accumulated over 400 additional hours since that experience, and am now ready to fly 121 without doubting myself"

Or something along those lines. If asked that question in an interview, be positive about the experience. And be confident that you are beyond it. If you want you can mention some personal family issues that arose during training that added to your already full plate, but you need to be careful so they don't think your trying to make any excuses.

-Imp