Why do I have such low self control?!!

I promised myself I'll maintain a balanced diet from today since I'm extremely bloated and also trying to conceive. I downlaoded fitness app again and added my meals with exact measurements, it made me feel so proud. But my bitch self had to show up and for no reason I felt like I needed cocacola and I'd die if I didn't get it... I drove through a fast food and asked for a large drink 😭😭😭 fml!! i couldn't even pass one day! I gained 15 pounds in last 2 weeks but still on healthy weight...well my parents already started calling me obese 😒 .... how can I force myself into a proper diet and stop drinking coke? Why is my motivation dead?!!!! There's two weddings I'll attend next month as well as my 3rd wedding reception in my country and I just can't stop my fat mouth from drinking junk 😣😣