Am I depressed?

I know it's a bizarre question to post about in a depression forum so do forgive me... But here's a little backstory: I started observing waves of loneliness and emptiness starting at the age of 14-15? I'm 26 now. I used to be relatively extroverted before then; active in sports and took the role as class clown. Not sure what happened when I turned 14 though. I can't really remember much of my emotions between the age of 14-19 other than the fact that I became introverted with a strong preference to be alone. Maybe it was some sort of "emo" phase; I've no idea..
I joined the military at 19 and it wasn't the best place to be "depressed" since nobody took it seriously and brushed it off as simple mental weakness. I left 2 years later. But even back then, my "depression" didn't hit me as hard as it does now.

I started feeling a sense of utter worthlessness after leaving the military; unemployed, with job interviews heading into dead ends despite looking promising; I had absolutely no clue what to do with my life. Now back then, I had a REASON to feel depressed.. I mean, who wouldn't be with all that crap going on, right? I never explored the possibility of actually suffering a mental illness until now.

As of 2017, I'm employed and earning a decent living (yay!). And honestly, I haven't got any logical complaints to make. But then the night comes... And I get this overwhelming sense of utter despair; it starts feeling like I'm alone in a dark enclosed cellar; everything starts looking meaningless, and the only effective thing I've found that helps with that heart-squeezing despair is to lie on the floor, curled up in the fetal position, staring into blank space or closing my eyes.
I've moved to a much healthier diet, and I spend an hour workng out daily - it doesn't help. I find myself moving from the push up position to the "hugging the floor" position, because well, maybe the floor'll be my friend and bring me comfort! (it unfortunately doesn't). I occassionally also sit on the floor in the shower in the fetal position covering my face with my hands.

I'm here now asking for advice because these feelings don't just come at night anymore; it's starting to surface during the day while I'm at work - which is unacceptable as I take my work seriously.. I've found some relief in covering my head with a hoodie, or heading to the restroom where I sit in a cubicle; hands covering my face, waiting for the despair to pass. It's getting worst, because I've found myself looking into the mirror; considering slitting my throat with a fruit knife. If death is empty, wouldn't that be better than a life filled with despair? And the pain of suffering a physical injury, wouldn't that make me feel alive again? I don't even know how it's possible to feel despair and emptiness at the same time.

I'm not really sure what to do, because it goes from 0 - 10 real fast. And that's just it, it's not a constant occurence; it seems to be largely random these days.. Any advice would be very much appreciated..