Comparing myself and Workout advice

My sons father and I split a while ago. During that time he was with other women. We are now trying to work things out. I keep finding myself comparing myself to them. They’re beautiful women with conventionally attractive bodies. He says I’m the most beautiful woman he has been with and that he finds my body more attractive and more his type. I know I should believe him and trust what he says, but I feel he’s saying it to make me feel better so the thoughts remain in my mind. I know it’s not his responsibility to make me feel secure. My biggest insecurity is my butt due to comments made from girls in high school saying my butt was little enough to hold with one hand and that I have a pancake butt and comments from family saying I have the family butt (all back butt). These comments from high school have stuck with me. I feel like my partner would be more attracted to me if I had a bigger butt. Is there any advice on how to stop comparing myself to the other women? I also recently started the gym to hopefully gain confidence, but I feel like I have no idea what I’m doing. Is there any advice on what I could do to grow glutes, minimize hip dips, and create a more round butt? I attached pictures to give an idea on what muscles I would need to target the most. Thanks in advance.