On Phentermine, still having cravings and appetite

I started taking Phentermine about 2 weeks ago, when I took it the first 3 or 4 days I felt the difference. I’m 19 (F), 256 Ib. and I also have narcolepsy. I think the first day I took it without and the second and third I took it with my prescribed narcolepsy medicine, Modafinil. I was feeling some really bad headaches so for the next 3 days or so I stop taking the Phentermine. Saturday was the first day of me back taking it, I took it Sunday and today as well and my appetite was out the roof for all days, including today! Should I just continually take it everyday without missing a day? I’m on 37.5 mg. Should I try taking it twice a day when I feel like it’s not working?