How do I stop masking when it's so ingrained in me as a behaviour?
Personally, masking was more of a confidence issue than anything else. Years of trying to hide the way our brains really want to work creates these awful negative feedback loops that become deeply ingrained. Eventually, you stop realizing that your brain works differently, and you become accustomed to your inability to act "normal."
Rather than exploring better ways of doing things, you explore better ways to hide your ADHD symptoms. It starts to become obvious that you really do suck at acting "normal" and your confidence starts to erode. But this is only logical. You actually do suck at acting normal! You have ADHD. Your brain doesn't like working in certain ways.
What you need is confidence.
Confidence that the so-called "weird" ways you prefer doing things is acceptable. More than acceptable actually, these are non-negotiable.
When I started doing web-development consulting, one of my clients would call me on the phone without warning to discuss certain parts of the project. You can imagine how disruptive and distracting these random calls were. After the third time, I calmly told my client that I could not work this way, and that I preferred emails or scheduled calls. Without missing a beat, they said "sure no problem" and he started emailing me multiple times every day with various different requests. This also became overwhelming, so I had another call with him where I confidently explained that I worked better with fewer emails, spaced farther apart. Once again the client quickly agreed, and we've been doing that ever since. It turns out he also preferred to do things this way, he just thought phone calls were how things should be, but he couldn't even remember why he thought that.
It seems simple in hindsight, but a few years ago I would've tried to pretend that these sudden calls and flood of emails weren't a problem. I would've tried to mask my problem with them. But those things didn't work for me, and it turns out that people have no problem doing things a different way if it works better for you.
After a while of boldly asserting my preferences, I became much more confident and upfront about communicating my needs. My relationships, future clients, friendships, and even hobbies became much better once I was able to ingrain in myself that my preferences are non-negotiable. If someone has a problem with my requirements, I can now confidently stand my ground whenever someone pushes back. If it becomes an issue, I move on, because keeping my brain happy is more important that someone else's preferences of how I "should" work.