Why is it SOOO dang hard to drop the pounds?

I had my baby back in January this year and during the time of pregnancy I got up to 200 pounds. I lost after having him but not enough. I went to my dr and he put me on phentermine diet pill I’ve been taking around 2 months now. I only lost 7 pounds last month. To some that may be a lot but to me ... I could’ve done better. Despite all of the healthy foods I eat, exercise and the adipex I take..my body seems like it just clings and doesn’t want to let go of the fat 😂😫😤