Possible depression

When you feel lonely, it can make you feel sad and bring you down quite a bit, which can be confused with depression. You're basically saying you began to feel this way because you feel lonely, because you're not in a relationship. Do you think you'd feel better once you would be in a relationship? When it comes to depression, even a relationship wouldn't take all those negative feelings away. It sounds like you just miss it to have somebody you can be with. There's no rush to find someone to be with. Let love find you, within the right time. Nobody can tell you whether you have a depression or not, you'd need to get a proper diagnose for that, done by a professional.

One thought I've always liked is that no matter what state you're in it is able to change. So if you're feeling this way now you still have the ability to feel better! And so for me personally if I start feeling terrible than I try to focus on helping other people, the more I focus on me the more imperfections I see, but when I focus on others it gives me the chance to do something good and usually that helps how I feel. Hopefully that helps, message me if you ever want to talk! Stay strong!

@little-angel-2 It isn't just loneliness anymore. Back then, I still ate, slept and got things done normally. The change happened about a month ago. I am lonely and I'm not in a relationship. For me, the two are inextricably linked. While I know that a relationship won't solve all my problems, I'm convinced that I would be much better off if I were in one. I wouldn't be lonely anymore. When it was just loneliness, I felt empty. Now it feels like there's a dark cloud inside me. The loneliness didn't get in the way of my life. Before I turned 19, I was happy with myself and with my life. Now it seems that I won't be happy until I'm in a relationship. I wish I could go back to the way I was when I was 17. I was confident, in control of my life, and not desperate for a relationship.

It sounds like it could be situational depression that you are going through but as Rain said, we can't give a diagnosis here and it might be helpful to seek medical advice from a proffessional. Even if you were diagnosed with a mental illness, that diagnosis is only a label (a tool if you like)which makes it easier to treat and it wouldn't define you, nor would it necessarily mean that you would have to deal with it for the rest of your life, depending on the diagnosis.
http://www.charlotte-anxiety-and-depression-treatment.com/situationaldepression.html
Hope things get better soon!

Yes. You fit the majority of the symptoms, and depression is extremely common. This isn't just loneliness or feeling blue. This is something that is bad enough that it is affecting your daily life, to the point that your emotions aren't functioning regularly, you can't be happy with things that used to make you happy, and you aren't able to perform important tasks properly, even ones that will affect your life in the future.

Sometimes depression is genetic. It can be caused by any change in a person's life, whether good or bad. It can stem from breakups, from moving, even from a drop in grades. Sometimes, it just happens without any specific cause. And that's okay. It's not less legitimate if it wasn't caused by something you'd consider particularly traumatic. It can happen to everyone. As long as the feelings last for over two weeks, it's depression. And this has been going on for MONTHS.

The thing about depression is, it can be caused by practically anything, and you can't just fix it with "a good attitude and a first aid kit". That's not how it works. Your brain is literally ill; just like any illness, you require some form of medical attention, be it therapy, medication, or hospitalization. It is illogical and improbable for you to tell yourself "happiness is a choice", because you're not choosing to be unhappy. Just like if you broke your leg, it would be sheer stupidity to say "I'm healed" and then go run a marathon. It's the depression, and mental illnesses are not controllable through sheer will.

I would suggest going to see a psychiatrist and get this diagnosed. You've got it, now you just need to make it official so you can get yourself treated. It is not a good idea to deal with this on your own. I'm assuming you have no experience in treating those with depression, so you aren't qualified to treat yourself the way an actual psychologist can. What people don't realize is that depression can be fatal; if it goes untreated long enough, if it gets serious enough, you are at risk for suicide. You need to get this checked out.