15 and worried

I'm not really sure if this is a question, more of a realization or a statement. I'm a 15 year old girl and I think I may have depression, but I can't tell whether I'm just jealous, bitter, sad, angry, helpless, tired and irritable all at once....and that's not depression, surely - that's just the average range of human emotion.

I have felt so helpless over the past few weeks - and this may sound horrible - but I think I've become this way because of other people. It's not that they're door anything to put me down, it's just that they're all so happy. They have successful relationships and plans for the future and bestfriends and things to do; I just feel like I deserve that, don't I? I feel like I'm invisible; I've stopped eating often, I've felt the urge to cut, I've attempted burning myself with a lighter, I've stayed away from social activities, I've tried smoking and I've cried for no apparent reason.

I sound so bitter when I read this back to myself, but I feel I need to write about this or it will never go away. But, from my understanding of these things, they don't just...go away. What worries me is that I'm either seriously depressed and am brushing it off as a stupid phase, or that I am not depressed at all, but am just amplifying my situation. I don't know if I'm depressed and the only solid evidence that I am comes from the results of my online 'diagnoses'.

In conclusion, I'm not sure if this is a valid question, but, how do you tell if you're truly, legitimately suffering from depression? Or can you just be experienced a short bout of it? Can it be brought on by certain life situations?

Any answer is welcome, any opinion, I don't care what it is. Thanks.

[SincerelyYoursT]

there are numerous forms of depression that vary in degree… I have struggled with forms of this personally. I define it as: "you know your depressed when you feel unhappy and hopeless, even if you had everything you want and things are good. When there is nothing that could lift your spirits…"
Feeling sad, lonely, unfulfilled, lost, confused, misunderstood, envious, insecure, frustrated, discouraged, hopeless…. are typical for your age (the age where so many teens turn down the wrong path).
For the most part, Depression comes on subtley… it is the way people react to it (eat, cut, drugs, isolate, etc) that launches it into a  vicious cycle (the more sad, the more you eat, the more you eat the more sad). The further into the cycle, the harder to get out…
What is the attraction to harming yourself (cutting)? Is it just to feel something???
I wish I reached out when i was your age…
Before you get into the self harm, talking to someone (therapist) would be really smart…
just remember… these feelings are not forever and do not have to get the best of you.
Stay strong and keep things in perspective (you know cutting offers no solution). Things always work out in the end-