Let me preface my doubts (in the heading) by saying that I am not relating them. I completely understand that depression is a disorder, a chemical imbalance in the head, and sometimes the symptoms can be one not taking care of themselves.

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I've (F22) been showing symptoms for years. I never got a formal diagnosis, on paper, but I have gone accessed free programs which offer to explain what your symptoms mean. The most credible "diagnosis" I received was this year, by a local hospital program (let's name it Project C) which offers a free initial diagnosis and referrals to professionals. They mentioned that they see clear signs of Dysthemia and/or Major Depressive Disorder, and OCD. 

My question is, how exactly do I know that I didn't lie to "not be caught", whether knowingly or unknowingly? As a child, I didn't always feel sad but I would make excuses whenever I didn't want to do things. Like when I was doing Project C, some of the questions were like 'I constantly feel sadness, at least half the time' and 'I find it difficult to find joy in things that I used to find joy in', to both of which I answered yes. How do I know that this is due to a chemical imbalance, or it's because of other reasons like hey I'm sad because I don't like what I study, or maybe I just grew out of the hobby that I once used to adore? Sometimes I suddenly cry out of nowhere, but I would have reasons. How do I know for sure it's because I'm mentally ill, or because I worry about things beyond belief? Because by the way I'm being told that my daily routines show a lot of indicators, I might've been depressed from the age of 12.

I let my room slowly become trashed, say, over a week. I can't stand it when I see small specks of dust/trash, so by day 6/7 I'll force myself to clean my room. I think the worst that I'm truly ashamed of, is I find it painfully difficult to get myself to brush. I can shower for hours, but I can't brush my teeth. I despise the taste of mint. I've changed toothpaste, it works for a while until it doesn't. Am I mentally ill here, or am I being lazy? How do I know for sure?

I annoy my brother to be on top of his work because education is vital especially for my family in this society, but I can't get myself to watch 5 minutes of lecture. Yet I sit and procrastinate, stressed to the core, wondering how I should've started earlier. Sometimes I wish my mind would just shut up, ever since I was 17 it has somehow lost it's pause button of some sort.

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Sorry went on a tangent there. But yeah, how can I be sure? I understand that I did get a diagnosis from Project C, but how do I know that all of the symptoms that I portray is because of depression/OCD, and it's not just me?