Lanky and working out

Hi im fairly new to the concept of going to the gym. I started going just before christmass when i weighed myself to find out that at the age of 18 and being 6ft 3 i weighed just under 10 stones!!!

I went to the gym and started to eat a lot more, although i never really ate much rubbish i cut out all crap, crisps, chips and sugar. replacing it with eating a lot of fish, eggs, ham and fruit and vedge and bran flskes. Since christmass i have been going to the gym about three times a week and eating more and using protine shake about 40g of protine acording to teh back of the can thing. i now weigh about 150 lbs and i can defenatly see a difference in my chest and bycepts.

The problem is that no matter how much i do on my abs i cant see much improvement i have a 4 pack when tensed but i havent seen my lower abs yet and i havent seen any noticable improvement for about two months. i do weighted situps crunches, let ups, planks and these things where you twist on the way up.

I also do some cardio and cycle about 20 to 30 miles a week and have a relatively fast metabolysm. i just dont know where to go from here i really need to get a six pack within the next six weeks as i am going on holiday with some of my friends and my ex will be there who dumped me cause i "didnt have the body" and i really want to prove her wrong just for the sake of it and teach her a lesson for being so shallow.

I just want to know if it is possible to get my sixpack in this short a time without using steroids or other chemical stuff? and also how i would go about getting my six pack.