I didn't really understand the line between NT and ND stimming

I would be careful to lump ND stimming together like this. Autistic stimming is different from ADHD stimming. To compare autistic stimming and NT stimming, the difference is that autistic stimming is much more varied and expressive. There's a complexity and depth in autistic stimming that is missing in NT stimming. Autistic stimming can also be hyperspecific. For example, I have a vocal stim after I've transferred GBs of data from a drive to another drive where I say, "There, the young ones are all aboard." I have a weird stim that triggers when the Internet is slow where I start blinking my eyes in a weird rhythmic pattern. There's probably at least 50 unique stims that I do and a lot of it is just weird shit like blinking-when-the-Internet-is-slow stim.

Not bad. I just have this one particular thing I do a lot, that I need to quit. Hmmm, lets try not doing that for tonight and note what happens.

In general, you can't stop doing a stim. The best you could do is find a similar stim and do that instead. And the similar stim has to be within the same category of stim (motor, vocal, tactile, and so on). So, if you have a maladaptive stim like headrocking into a hard surface (ie you're basically giving yourself a mild concussion), you could switch that stim into something less maladaptive like headrocking into a pillow or headrocking in the open or slapping your forehead or punching things, but you can't really just stop doing the headrocking-into-a-hard-surface stim cold turkey like that.

I don't understand why I am able to not do it for a few hours/all day if I'm out but when I'm alone I need it.

You're masking when you're out, so to compensate, once you're alone when you don't have to mask, you stim more to make up for the times when you weren't stimming because you're out in public. For me, I feel suffocated if I'm forced to not stim. For work, I get around this by stimming during bathroom breaks. You could even vocally stim to a certain extent if your vocal stim is humming. Driving a car is also a good opportunity to stim because it's not like people are going to hear your vocal stims or see your weird hand flapping while out in the road.