I've dealt with anxiety my whole life. When I was a little kid summer's off of school were fun. When I hit puberty things got weird, when I was about 13 during the summer months off school I became a hypochondriac that summer, although I didn't know what that was at the time. Went to the doctor, and I actually did feel sick, they didn't know what was wrong, they put me on meds that did nothing, but now I look back and can see that a couple of months off spent hanging around the house means your mind needs something to do anyways, and when it's not busy it will find something to think about, hence things like hypochondria and OCD. When I went back to school in Sept i was fine. As they say, a rolling stone gathers no moss.
I have a very active brain, always thinking or dreaming about something, i'm very analytical and creative, which is both good and bad, because if I don't manage my thoughts they can easily ruminate on negative things, or my mind will simply be too active.
My dad is the total opposite. He uses his brain as minimum as possible LOL. His mind is almost always in the present focused on the task at hand, he's extremely practical, he's not daydreaming in the clouds or worrying about this or that. He's extremely mentally healthy, and very laid back. I can be neurotic about things, him not at all.
A lot of anxiety is a fight or flight response (anger and/or fear), but some of it is your mind simply not being in the present. That's why meditation is so helpful in my experience, it's more effective than any drug IMO, but more annoying because it takes a bit of time and work.
Been lucky to not have much depression. Some of it like probably our friend ColinBlow is a genetic imbalance. The times I've been depressed is when the reality of my life doesn't match my expectations for being happy. ie: If you're a 45 year old woman and never had kids, but always wanted them, that's when you can sink into a depression. So you either have to change your reality or change your expectations or you'll never be happy.
A therapist also once told that if you try your hardest, you won't be depressed. That's probably an exaggeration, but it's great advice. If you sit around all day doing nothing, knowing you probably should have done something productive, that can make you depressed. Generally living life with no regrets.