As others have noted, while a safe weekly weight loss would be 1-2 pounds per week, there may be weeks you don't lose. there will even be weeks you gain ! And that doesn't mean you're doing anything wrong. try not to get trapped in the dieting mentality. Instead, do your best to eat right, watch your portions and get some regular exercise that includes some strength training. Will that help you fit into the dress by May ? Maybe.
However, I DO know a way you can fit into the dress if you don't lose 10 pounds. Go to Macy's or any good retail store that has a lingerie department. You're looking for SPANX.
Spanx is the brand name of a pair of panties (your mom might call it a girdle) that can help take 2-3 inches off your waist. Many brides wear something called foundation garments under their gowns. these garments can cinch in waists and take inches off your hips. Shoot, even JLo wears them to smooth out the lumps and bumps. www.dailymail.co.uk/tvshowbiz/article-2216
The point ? If you don't lose enough to fit the dress, there are undergarments that can help you. it might not be overly comfortable, but lots of women use these undergarments. And they're not too expensive. Actually, you might ask your mom if she has a pair.