My answer is below in spoilers
You never stop trying to tear me apart
Without a thought, not a care in your heart.
Can't say I quite get this, but may refer to constant pressure against a tight belt. Stolen from Dan Russell's answer: Tearing apart is taking it off. I guess the you never stop trying part threw me for a belt loop.
Sometimes you stab me with iron or steel
I just hug in return, although I'll never heal
you can puncture new holes if a belt is too loose. Hug is wearing the belt. And I suppose just putting the belt on could entail stabbing in a simpler form, but the heal made me think in the creation of holes.
I know, I know, you just want my support
Which I will provide, with no retort.
it supports your pants
You used to scare kids with me back in the day
At least now you can't - it's no longer okay.
Beating kids is bad, mmkay
All tightened up, I wait in suspense
I cannot relax, I'm constantly tense.
Once more the belt is tight, with a possible reference to their brethren suspenders.
My only relief in a day so bleak
Is when you leave for a second to go take a leak
Undo the belt to take a leak
When you're formal and proper, I'm stressed to the max
I want this to end, can you cut me some slacks?
Slacks do be pants you would wear a belt with. And when you are trying to look fancy you must be prim and proper entailing a most vigorous tightening of belt lest gravity aims to pants you with much impropriety.
The answer may hide if its genuine:
Specifically it may cowhide if it is genuine, or be made from leather.
I hate owls! Wait... no... that doesn't look right.
Working on it...