Plateaus suck. I'm going to disagree with Omega. If it wasn't important to you, you probably wouldn't post.
OK...so I'm not telling you how to fix it...I'm telling you what I would do if I was stuck on a plateau.
First of all...I wouldn't panic. Anyone that's ever dieted has been there.
Second of all. I wouldn't stand for it. It would be unacceptable for me to be on a diet for 3 months and not lose any weight. I want to see progress, and I will, by God see progress, or I'm going to fix it.
Third, I'd realize that there is nothing keeping me from losing weight but me.
Your reported exercise levels and caloric intake puzzle me greatly. Of course, I don't know your size or circumstances, but it seems like that's not many calories and a lot of exercise.
Are you sure that's really 1500 calories a day? Is that every day or on your best days or what? Has your portion size blown up on you (MY #1 issue in this area has always been portion size).
On the exercise...I'd suggest working out harder. See a doctor first and all that. But I personally work out 30 minutes a day and have dropped 101 pounds, as of this morning, in the last 14 months. But I work out absolutely as hard as I can WITHOUT putting myself in a position that I can't force myself to go again the next day. If you're not sore....if you're not feeling the burn....if you're not soaked with sweat....if you're not gasping for breath....than it ain't "cardio" in my book.
Good luck. I mean it. Really. Good luck to you.
Phil