man, the same thing that happened last time will happen again -- you'll lose x amount of weight whilst you take the phentermine, then put that same amount of weight back on in a short amount of time, and possibly even more.
I've lost 15kg (33lb) using meth years ago, and it took 6 months to come back.
Then I also dropped 20kg years later by exercise, good food, and gym. And ya know what? It stayed off.
Any stimulant you take will only supress appetite. And when this happens, your body goes into 'starvation mode'; it isnt getting enough calories for food so your metabolism slows down, making burning off calories harder, and muscle is broken down in preference to fat. So when you come off the meds and start eating like normal, your body is still in starvation mode. And when its in this mode, an excess of calories after so long of not enough calories, makes it so you stack on fat really quick, because your body thinks it's best to store energy (as fat) now while it can.
Any drug that acts on appetite will only work in conjunction with exercise, and a proper diet. A diet which provides enough to keep you losing waight, but doesnt trigger the starvation mode.
The absolute best thing I have ever used is called DNP. The body building community uses it, but it is an extremely dangerous compound to use, as an OD will definitely cause death due to overheating. But look it up, 2,4-dinitrophenol. Bodybuilders can lose up to 1lb of fat per day . The DNP uncouples the production of energy (ATP) in the mitochondria in cells; hence, energy is wasted as heat, and the loss of heat must be matched with energy production from breakdown of fat, and only fat. DNP is just too dangerous for newbies though. Luckily, there are other less toxic drugs which work in a similar manner (though less effective) and I would suggest these over DNP since you dont know what youre doing. Usnic acid is one such compound which works to make you lose fat without putting you in starvation mode. You should find usnic acid online, as like DNP, it's not a sceduled drug or anything like that.