Unfortunately its quite simple.

Your body has a resting metabolic rate. This is the amount of calories your body needs just to exist. Doing exercise will increase this figure.

To lose fat the calories your body uses has to be more than what it has available.

So, something like :

Calories for Fat loss = resting metabolic rate + calories used as exercise - food eaten.

So, assuming your body needs 2500 calories.

300 = 2500 + 300 (assuming daily significant exercise) - 2500 (assuming you eat "normally").

So you have 2100 calories for fat loss per week (7*300). Considering there are 3500 calories in a pound of fat, it will take you around 2 weeks to lose a pound of fat. Thats all very simplified of course, but I hope you can see where I am coming from.

Of course all of the above depends on how much your actually eating, assuming more than what your body needs as you have put weight on. I dont think you have a choice but to cut down on what your eating. You have kind of implied you do all sorts of exercise, but your waist isnt getting any smaller. That implies your not losing any fat. Maybe muscle if your weight is going down.

Unfortunately you dont have much choice but to eat less. You have admitted yourself you wont keep to a calorie controlled diet. Why is that? Maybe because you dont want to enough. In that case in 14 weeks time, you will look much the same. Can you make sacrifices for 14 weeks to get what you want? At the moment it seems you want to eat/drink what you like MORE than looking better for holiday. Is that the case?