You have to plan a proper diet chart. It is very important to have a restricted calorie intake after the age of 20 as your growth is complete and any extra calories will result in your present problem.
You also have to measure your BMI and try to achieve a BMI of less than 25 and also try to understand your basal metabolic rate.
Try different forms of cardio for an hour regularly. You can do treadmill, elliptical, running, jogging, skipping, cycling or swimming.
Don't get down - that's the first step! Feeling low about not being able to lose weight can influence weight loss (believe me, I'm going through that right now and have battled with weight my entire life!). It's possible you need to vary your diet a bit more - I've just said the same thing to someone else who had a similar problem.
When you start a diet the weight drops off over the first couple of weeks because your body is readjusting to a new 'way of life', less calories, less saturated fat, more exercise. After the first couple of weeks your body is starting to get used to this weightloss regime and so the weight begins to take longer to come off, and comes off much less than it did before! I'm at that stage now - I started at 12 stone when I was just 13 years old, in my first WEEK at Slimming World I lost 7 pounds and got excited because I thought it would happen every week... but it didn't. Now I'm 27 years old and barely lose a pound a week!
You need to make sure you're not eating the same thing, day in, day out. Vary your meals as much as you can. Drink lots of water - especially if you're exercising a lot. Some weight gain/staying the same weight can be caused by water retention and drinking more should help this. More exercise isn't always the answer (although it can help, of course!). Variation is often the key to success - your body can get just as bored with the same foods as you can!
I hope this helps a little bit. It sounds like you're doing well so don't give up, don't starve yourself (because that really won't help), and add a little variation to your diet every week and you should start losing a bit again. Just don't expect the pounds to drop off as quickly as they did to begin with! But they WILL come off. If you're having problems with finding different foods to eat then talk to your Weight Watcher's adviser. They're usually really good with meal planning and should be able to help you :-)