Phentermine - I only lost 13 pounds my first month. How can I increase my weight loss in the second?

Three (3) lbs. a week is INCREDIBLE, but probably not too healthy for one month. Taking pseudo speed is a short term answer to a long term problem, yes?
The only way other than eating healthy is to MOVE more than you have before dieting.
I've found working up to tread milling AT THE GYM for 60+ minutes 3-4 times a week has my BMI now normal @ 25. Give it a try!!! I emphasize at the gym because exercising at home seldom works. Making a trip to the gym does work! Use an iPod to keep your mind off the time and put a towel over the read outs when you treadmill.
Remember weight loss is simple - Calories eaten minus calories out. You have to eat less calories than what you burn off in a day WITHOUT exercise. The only thing that ALSO affect weight loss is EXERCISE preferable aerobic exercise. Good Luck and all the very best to you!