Is 1200 calories really realistic?

Basic info- f, 37, current weight 260. Prediabetic and hypertensive. I lost 30lbs on my own between July and November last year and have been on a plateau.

Just started Phentermine as prescribed by my gp. I wasn't given much guidance just a generic goal of 1200 calories a day.

I have tried but can't get my calories that low. I typically walk for 30-45 minutes a day or do a fitness class at the gym.

Edit- wow, I don't have hypertension I have high cholesterol. That was silly of me. On meds for the cholesterol and Metformin for the prediabetes.

I am 5'4"