Borderlands – Is the video ‘Krieg: A Meat Bicycle Built For Two’ an official part of the Borderlands Lore/Continuity

The "Mastering X- Y" you refer to refers to two things.

X will refer to what Playthrough that player is on. Whether it be, 1, 2, or 2.5. Y, on the other hand, will refer to what quest of the Main Storyline that player is on. Since there are multiple quests that correspond with each playthrough (including sidequests mind you), these quests are also labelled with numbers.

In your case, "Mastering 1-7" Would refer to Playthrough 1, Hunting the Firehawk.

when you are at the main menu hit Y if you are on the xbox and re click to load the character you have already loaded. When it reloads it will ask you if you want to play true vault hero mode or normal mode. Click normal mode and you will be back to play through one and have your situation fixed.