If you actually believe that at 124 lbs and 5'9" in height is an overweight person, then you have some serious self image issues that need to be addressed. You are at what my weight should be at 5'4" tall.
I highly suggest you try to build your confidence level so that you are able to accept yourself for who you truly are and be able to love yourself just the way you are. I know the media is constantly pushing thin beauty down our throats, but that is not real life stuff as it is all about selling products and services.
You need a confidence booster. What activities do you love participating in? Seek out the things you enjoy and you will be surrounded by like minded people with the same interests where you can have a good support system of acquaintances, make new friendships and maybe even find romance along the way.
It is typical for 18 year olds to feel a bit insecure as they find their path in this world, so your feelings are normal.
However, eating only one meal a day is dangerous for your internal organs, and above all your skin, hair and nails. If you continue on this path of starving yourself, your looks will fade rapidly and you will lose the sparkle in your eyes. Your skin is your biggest organ and it often is an indicator of your over all heath. If you continue to starve yourself and rob your body of the nutrition it needs to grow and be healthy, you will diminish your immune system and begin looking somewhat haggard. At 18 years old, your brain is still growing and will not be fully developed until you turn 25 years old. For crying out loud, you are still growing teeth and need nutrition.
Do not skip meals because that slows down your metabolism and actually causes weight gain. Really, look it up. You need to eat breakfast, lunch and dinner. Just eat healthy foods like fruits, vegetables and lean meats and fish along with whole grain breads, rice and pasta. That is a healthy diet that can help you maintain your weight and keep your organs, skin, hair, nails and teeth healthy. Snacking on healthy foods throughout the day is a good way to maintain high energy and increase metabolism. Read up, you will see that I am right.