The answer is:
A belt
Is this about all the heroin?
Heroin refers to belting an arm before shooting up.
I don't just mean keeping you clothed and respectable looking.
Clothed and respectable as belts suspend pants and are classy as can be.
I don't just mean saving your life.
A (security) belt can save your life.
Everything ... from my tongue reflects you
The toungue of a belt is usually metallic and reflective.
Now you say you don't need me? ... remember this conversation when you come crawling back. And don't until you lose some weight.
The belt is no longer needed because the wearer has gained weight and their pants now stay on. Notice that the "and don't until" bit is written in such a way that it could mean that he/she won't decide to come back till there is weight loss and the belt is needed again.
I'm not ending up at my limit again
The limit is the last punch hole of the belt.
Thanks to the OP for his feedback on this answer. My old answer is included under the line.
My guess is:
A security belt
It's all about the heroin
In this context, the heroin is a person belted up
I don't just mean keeping you clothed and respectable looking.
A security belt is not just something you put on like clothes.
I don't just mean saving your life.
A security belt can save your life but it can do more.
I've been firmly with you everywhere you go.
A security belt is tight firmly to the passenger.
Everything ... from my tongue reflects you
The toungue of a security belt is usually metallic and reflective.
Now you say you don't need me? ... Go off
When you put off a security belt, you don't need it any more.
And don't until you lose some weight. I'm not ending up at my limit again for some hot mess like you.
Makes sense. If you're fat, you're going to pull the security belt to its limit.
Update (to the old answer)
After thinking about it once again, the solution could be also just:
A belt
This would explain the "ass" part better, although other clues would then sound more fanciful.