How to gain weight

I am a teen and I am very underweight. I used to have an eating disorder where I would barely eat and do a ton of cardio. I was scared if getting fat because my mother always told me men won’t like me if I’m chubby. That was 6 years ago. Currently I gained some weight but I’m still underweight. Some of my chest bones are visible and same goes for my ribs. I want to gain weight but I find it challenging because I think I will get fat. The weird thing is that I was weighing 101lbs last month but now I weigh 94lbs. I don’t know how I lost all my progress. During that time I was also doing weight lifting (middle school/ highschool) but since I never ate I never gained weight. For some reason I found some stretch marks on my thighs and I’m now even more afraid of gaining weight. Please share some advice because for my height and age I’m very underweight. I also don’t know what foods to eat. I just want to gain weight and not get a big stomach. Since I never ate when I was young, my body didn’t grow up either so I look like I’m 12 even though I’m in my early 20s. The reason I want to gain some weight is because this guy I was seeing commented on how I look fragile and how my chest area is so small, and that has made me even more self conscious.