Not encouraging and motivating yourself to educate the path in which you want to walk.
And, not examining how the basics apply to yourself:
1. Such as learning your personal calories, and how to manipulate calories and macros according your goal you desire (muscle gain/tissue loss).
2. Such as learning the basic mechanics of strength training/exercising in general, and how to manipulate the variables (sets, reps, tempo, rest between sets, progression, etc) according to your goal want, to combat personal problems encountered, and to solicit another kind of stimulus.
3. Not setting some time aside to study to learn numbers 1 and 2.
After losing 40+ unwanted lbs: