Simply put, adhd is the result of a signal substance deficiency in the brain. The car is out of fuel, if you will, making it very hard to start (initiate tasks).
However, the brain is very good at adapting, and can learn to harness other brain chemicals as fuel! The most common one is dopamine - the reward chemical the brain releases when you do something fun or interesting. That's why hyperfocus happens, and why it's easier to do fun or interesting (or new!) things than boring things or repetitive chores! Fun things supply their own fuel!
Sadly the brain likes being fuelled up, and tends to only turn its attention to Interesting Things rather than the boring thing you should be working on. 🥲
Another substance that can be hijacked to beat the brain dysfunction is adrenaline. That's why deadlines work! When you get stressed enough, the brain gets hit with a flash of adrenaline, which can be used as fuel. It is extremely common for people with undiagnosed adhd to come to rely on deadlines/adrenaline to get things done!
The bad thing is that 1: the brain needs more and more adrenaline to reach a level of function, so the more you rely on it, the less effective this strategy is.
And 2: adrenaline is, at the end of the day, a stress-chemical, and keeping your brain in a constant state of stress is highly damaging. You will suffer burn-out. It's just a question of when.
And once you've hit that wall once, your tolerance for stress will be shit, and the brain will start breaking down at even low levels of stress. Not only can you no longer harness the deadline-power to get things done - your brain will be in worse shape than ever before. Complete inability to focus, lack of memory-retention or even memory loss, brain fog, chronic fatigue etc. And that damage can become permanent.
It's a horrible place to end up at, getting the equivalent of dementia in young age because you thought you could keep pushing through because "it's always worked before". And sadly, it's very, very common.
On top of that, the risk for consecutive, damaging burn-outs is even higher if you're also on the autism spectrum, burning through extra brain-power just to keep up with all the sensory input and social cues.
So if you find yourself relying on deadlines to get things done... consider getting an adhd evaluation and see if you can get medication. It's brain fuel in a pill! It can really help!
And if you're unable to get a diagnosis or medicine for whatever reason (I know it can be really hard to get either) - at least make sure you include rest in your daily routines, low-sensory downtime where you're not doing anything. No reading or talking or tv or games. Just dark and quiet rest. Allowing the brain to recover and repair the damage of adrenaline stress.
Take care of your brain. You've only got the one. Be nice to it.