How to do this right?

So I’m in the process of loosing weight. I tried doing it how the nutritionist said and how my bf has been encouraging it but I physically can not. A lot of you guys said 1400 calories is not healthy to do to start off. I used to drink 3-4 sometimes even 5 pops a day. And I’ve cut it down to nothing the last few days. I’m having the most intense migraines I almost went to the ER last night it was that bad. Background I have migraines real intense. So migraines are common for me to get. I get triggered by lack of sleep or being stressed but I’m not currently stressed or lack of sleep but these ones are INTENSE and unBareable. The last time I’ve had migraines THIS THIS bad was when I was pregnant and all the hormones. I think it’s from no pop and going from so high of calories to barely any. So I wanna do this right what would you guys give for advice on cutting down to soda? I’m seeing my body isn’t responding well from going cold turkey so how do you gradually stop drinking it to where your body can function? Like 2 a day then slowly 1 ? Thanks for the help :)