Mach X's history is exactly the same as that of Earth-616's Abner Jenkins up until the Marvel NEW! relaunch.
Abner Jenkins felt accused of trying to forget the man he was all together. He proved the public wrong when he returned to his classic colors in his new Mach X Armor. He showed everyone he knows about what he used to be, but instead of erasing that image, he uses it as a motivation to be better. He started small, taking on tiny robberies and crimes near him. It wasn't until he thwarted the a team of Beetles all armored like he was that he showed his worth and was added to the Avengers