How to stick to my diet? (Weight Watchers)?

So weight watchers has helped me alot. In the past year and a half i lost 45 lbs from it! Which is good because I was 170! (I'm 16 years old btw). But now I gained back like 13 lbs which i am very upset about. Idk how i could let myself do that. But I'm done being upset about it I'm ready to change and start losing again. I used to be soooo strong and determined. What happened? Please guys I am begging I need some tips and advice to get back on track and kick some major as*! help me