Hi there!
I'm 22 as well, and I too am a student (taking my Dental Hygiene course) and know exactly how you feel about the previous diet attempts and losing so easily but then it packing back on..
I did WW in high school and lost 50 lbs (in b/w Nov to June, going from 199 to 149) I went to university and college and packed on the weight and my boyfriend (whom Ive been with for 4 years) hadn't realized how much I had actually gained. I started up with WW again in November and I'm really trying this time to pack good snacks and lunches for school so I'm not tempted... contrary to popular belief we Dental Hygiene students loooooove our sweets!! haha!
I'm really trying to make a whole lifestyle change and concentrating on my hunger cues.
I've never been on the southbeach diet but I know for different tips for the lunches (according to WW) I try to get whole grain bread *or english muffins* and make a sandwich with extra lean smoked turkey, a pickle, mustard, and a bit of grated cheese) it comes out to being about 4 of 5 pts (depending on how much cheese and turkey you put on) which is awesome! And then I bring raw veggies, pretzel sticks, and other little things like that for snacks. i'm still adjusting to being back on WW but I hope my small tips help!
Have a great week!