Gaining weight

Just been to the hospital on Wednesday night and found out that I am only 44kg (6 & 1/2 stone). I am 18 years old and 5ft 1in. I was told by the doctor who checked my weight that I am underweight and would need to bulk myself up to at least 52kg in order to be a healthy weight. However this is where my problem comes in. I have been trying to put on weight for maybe the past 4/5 months now and am barely noticing any difference. I did not know I was only 6 & 1/2 stone I genuinely thought I was maybe at least 7. I have been eating lots of fattening foods mainly healthy fats such as avocados, peanut butter & olive oil. As well as things like potatoes, sweet potatoes, various fruit smoothies (not shop bought), meats, rice, pasta & a lot of dairy products such as cheese and yogurt. But I just cannot seem to put the weight on no matter how much I eat, obviously I have been eating a lot of unhealthy foods as well purely in hope that it will help me fatten up. I hate the way my body is I am always mistaken for a 12/13 year old by people I meet & forever have people I know telling me that I should "be glad" that I'm skinny because most girls would "kill to be my size". I don't know why anyone would want to have my body. I have no boobs or bum at all and feel so unhealthy and childlike looks wise. So what I am asking is have any of you ever had a problem similar to what I am having and are there any specific daily diets I could try to help me gain weight? Please and thank you.