Calorie intake and P90X

I just started P90X today, am confused on calories that I need to eat. I lost 65 pounds in 2010 and gained 20 back this past winter, I want those 20 pounds gone by the time I go on my trip to Punta Cana in November. I joined MFP in Feb/Mar and can't seem to get the weight off no matter if my calories are 1200 or 1500 or if i eat my exercise cals back or not. I'm really frustrated....my doctor said its due to the fact that I'm 30 now so I'm going to have to deal with it or get lipo (nice huh?!) I carry the 20 in my tummy and thighs everything else is great. I went to a nutritionist she said I need 1200-1500 cals a day and to never eat back my exercise cals. She said I need more strength training and less cardio and I should see results. I burned 300 cals today doing day 1 of p90x I'm sure I will burn more once I get hang of moves etc. but if I burned 600 a day as the p90x guide says I will only net 700 a day that's not enough is it?! I want to get out of this plateau and lose the 20. But p90x says for me to eat 1800 that is a lot! Can anyone just please tell me what to do? I just need that lol...I will do anything to get this weight off!! Thanks in advance!