I face the same scenario, 54kg and 5'8...when I was 17 and used to work out I was at 60kg and now at 23 at the same height and lame lifestyle devoid of breakfast thanks to my sleeping habits I am at 54kg.
Smoking, late sleeping and just on time waking in the morning for work, lack of fruit intake(i just dont like fruits), not taking the proper 3 meals of the day all contribute to my health's downfall...
I know what I used to do right when I was younger and I know the solution to your problem but just cant be bothered to help myself with it.
- Sort your sleeping routine (i dont know if its out or not but you need to be capable of waking early for the breakfast run)
- Take a healthy breakfast can be anything you wish, oatmeal, cereals( pack a variety of them at home and eat any u feel like eating in the morning so that the taste may not get to you), eggs in any form except raw, fruits, milkshakes and juices should be taken alongside.
The moment you begin with breakfast I can bet you if you've had breakfast at 8-9am your stomach will be asking for something by 12-1 if you haven't slept in that period, you can at this time take in lunch or a small snack such as noodles, etc. at 2-3pm you can follow up with lunch. Meat is all important so dont skip on it. I assure you because of a proper breakfast your metabolism will be activated as it was intended to be by natures's course and you'll start feeling hungry for more food if you make your body receive food whenever you feel your hungry even if its not time for food. It shouldn't be anything we recommend it can be anything you like!
Once your metabolism kicks in buddy you could be seeing at 5-6 meals a day, dont ignore your body at that time keep feeding it whenever your stomach feels the need once you keep on doing you'll find yourself in a very pleasing situation following that...I cant follow such a regimen because of my work and because im not home most of the time, you'll need to thus shape this according to your own lifestyle.