How can I speed up my metabolism?

I'm 21, I weigh around 10 stone and I'm 5.7. I exercise about 3-4 times a week usually to Jillian Michaels 30 day shred (circuit training) its only around 30 minutes long.

I eat healthy, mostly protein and very little carbohydrates. I always seem to be on a diet but never really lose weight. I usually have an omelette for breakfast (1 whole 2 whites), a ham salad for lunch no dressing, and grilled chicken and prawns for dinner.

Whenever I have more carbohydrate I just seem to bloat and feel sluggish. I know I'm going wrong here but have no idea where?