In need of advice for an eating disorder

I saw a lot of posts on here of people with similar problems, and the support was amazing. I'll start off by saying that I am a 5'9" female. I used to weigh 118 pounds before I started lowering my intake. At first, I would only eat 800 calories a day. I began to limit myself, and now I am at 400 calories a day. I now barely weigh 105. I am trying to stop myself before it gets really bad. I'm too scared to even go out in a bathing suit because my ribcage can be seen. I am about to finish school this year and this condition gives me no motivation. I have been having many image problems since my childhood; I had a really hard time growing up. I felt like I was fat and I hated how I looked and that's when I began to 'diet.' I really do want to get back to a healthy weight, but I am so incredibly scared of gaining weight. I am trying to motivate myself, but I cannot get over 500 calories a day if I really try. It is so hard for me to imagine gaining more weight. I want to be healthy though. My goal is to try to get in at least 1,200 calories a day, but I heard you have to increase the calories slowly. I know it is still very underweight, but I want to at least try to get to 110 at first. I don't know what foods I should eat or how I can force myself to eat them. I just need some help before it gets worse.