Depressed, but i can't seem to pin point on what issue...

I've been feeling this for almost a month now.
I haven't been telling anyone about this matter. When i hang out with friends /family / colleagues....i can laugh and joke about.

but when i sit in the car and drive alone....i think of stuffs. Recent choices that i've made. Recent worries that surface....
I can't help but to feel helpless in situations like these.

for example, today was an outing with my group of very close friends, but somehow, i haven't been talking or responding much to the conversation as i always had.
I always feel that maybe i should just sit put and shut up. I try not to speak that much, and only listen.
There are lots of things that floats about in my mind, even now.

My relationship with my fellow not-so-close colleagues are getting worse, and my temper isn't getting any better.
When i reach home, i try to dash straight into my room, and keep myself couped up.
I don't watch anime, tv series or read manga or even cosplay that much anymore.

Everyday i worry about stuffs, but i can't seem to pinpoint on what.

Is this a phase everyone has to go through?
The 'maybe i should've done this', or 'maybe i shouldn't be doing that'...
I'm hitting the 30s in a couple of years, so is this like...a mid-life crisis?

Listening to : ------
Mood : worried (i dunno about what)