Are we even friends anymore?

It's also some kind of reflex of mine to say "I know" to everything or "I don't care" (although I've inherited this trait from a mean friend). What's keeping me in check is remembering the past or other mean people, like my parents or my former friend who bullied me. And also talking in an overly optimistic voice about issues that bug me. Helps a lot. You said you feel like you've gone numb, but isn't it more like you are annoyed when you say "IDC", which would make them stop talking? And because that annoyance is so prominent, you wouldn't feel other emotions as much?

I just remembered that one south park episode where Stan was depressed and everyone left him - and I think I might have brought across what I said wrongly. In case you are depressed (and therefore feel numb) which makes you mean which makes no one put up with that attitude of yours and which makes them distance yourself from you: They key isn't to put on a mask and change your asshole-attitude, thinking the problem is you. Instead, you'll have to find something that will improve your mood (actually, going outside, doing stuff I did in my childhood like letting the TV run in the background and staying away from social media helped me for instance.) and perhaps deep talk with them about it if you're close enough. Many people deal with life's shit and there's a chance they might relate. Even if they don't, they will still gain insight of you.

If you are more the teasing-kind of friend, and your friend gets easily hurt when you mock them or whatever, then you'd have to tune it down and respect their boundaries as they take in your 'meanness' differently. Maybe let out one or two teases in a while, but not all the time. I've noticed on myself how I'd become increasingly more mean to someone the more we are friends. That can quickly turn into toxicity or even bullying (but then again that's only the case for me, I don't know what you do).
I've also realized how that toxicity of mine derives from my negative outlook on life and myself since I degrade myself very often. So I don't think it's too far-fetched to say that I projected this negativity on my friend, making life even more miserable instead of sharing positive feelings. Plus I've been depressed which explains why I was being so mean.
I don't know if my experiences help you in any way. I'd suggest you analyze why you behave in a certain way and whether this can be tolerated or not. Harsh words can be taken lightly as a joke everyone enjoys, we are youngsters after all and that's typical humor among us.

You said you talk less with your friend these days and you feel like you have to control your behaviour around them. It's natural someone changes and they will eventually too. The feeling of "I know you" and feeling subconsciously close to them, even if you have less to share now should still exist though. Even if you have entirely different opinions on topics and differnet hobbies you can still stay friends. Adults do that all the time, though I don't know about the depth of the relationship. It should still make you happy anyhow.

The relationship shouldn't drain you, and I hope they aren't your only friend. You should talk to others too and even make new friends. Something like that would be refreshing and even let you figure out more about yourself.

Oh and, maybe ask them from time to time "was that too mean?" so they'll know you genuinely care about them.

I hope my response helped you in any way.