From my experience, if you keep declining yourself stuff like that, you are setting up yourself to fail. Have that pizza, just remember to not go and stuff yourself to the point where you can’t walk because you’re full. Remember to eat slowly too.
you can also just make small modifications to decrease the calorie density of your meals, adding veggies is great.
Also, life hack: you can eat basically as much fruit as you want when dieting, it's so filling (due to the water and fibre) that you're almost guaranteed to be full and probably quite sick of it before you consume a ton of calories.
See, I'm an all or nothing kind of person. "Just the one slice" doesn't work. I eat the whole pizza. I've had much more success with regimented diets. The one I've settled on, which has worked well for years, is intermittent fasting. I only eat in a six hour window and this basically means two satisfying meals of whatever. I have to try hard to overeat in that window. It also helps to cut down on snacks.