I no longer have much difficulty making and breaking habits. I also have ADHD, and have not always been so able. Here is much of what I've learned from asking questions like yours for the last 20 years:
- It helps to be as meta-cognitive as possible. I am still learning to examine the thoughts and impulses as they appear. This is an ongoing process that, to the best of my understanding, has no discrete endpoint. I use a formal-ish (but consistent) daily meditation practice to keep the meta-cognitive pathways strong, and I practice as often as I remember to. My go-to "in the wild" practice is a quick mind-body scan: how is everything right in this Now? I find it beneficial in those moments to invite areas of (mental and physical) tension to relax a bit. It makes me feel better, so I want to do it more.
The primary benefit in this context is that you will begin to see the excuses and outright lies that the mind invents to get out of doing things it anticipates might be like work. Brains are lazy creatures! You have the choice to believe, disbelieve, and act on thoughts or not; but only if you are aware of what they are (and what they are not) in the first place. This is nowhere near as trivial as it sounds! And it can only be accomplished by routine examination of the mind.
- Build systems and procedures that cause things to get completely done every time. I'm a big fan of checklists! Many of these live in my head, but there must be some kind of somatic component to the act of going through the list*. Paper and pen(cil) work well here. Checklists may not be for you! Try stuff until something feels good, then try to go with it. If some part of a system feels like a chore, throw that part out. If that breaks the system, throw out the whole thing and try something else.
- Example: On my way out of work I put my bag in the car and say "Bag", my lunchbox "Box", and my hard hat "Bucket" (alliteration, yo!). Then I check my pockets for things it would be easy to take home but I shouldn't. Each of these things has a dedicated location on my person where I will find it if I still have it. I name each thing: "Keys, computers" and touch where each belongs. Too many times I've saved an hour of driving this way!
- Export the leading ends of procedures to the environment. Related to TAPs, this practice gives me a concrete, visible, tangible place to start every single time. I build daily routines around interactions with my partner and family**. My work routine is contained in a checklist on my phone (that I made more amusing by writing it in a phonetic alphabet, coloring it brightly, and structuring it in three acts, all so I would want to look at it a little bit more. YMMV).
** Example: My evening routine is maintained between me and my partner. We brush teeth and the cat, dress for bed, sit a short meditation, and maintain hair together in that order every day. It's actually kind of disturbing when circumstances cause us to sleep in different locations for a night! But that occasional discomfort is well worth it so I remember to floss nearly all the time!!
- Don't be afraid to fail. Popular methods like "Don't Break the Chain" suggest that habits are an all-or-nothing proposition. But the truth is that with that kind of attitude, a single "miss" can generate a sense of failure that breaks the habit entirely! Nearly all of our "daily" habits can be accomplished "daily-ish" without losing much utility. The important thing is to go back to doing the thing on the next opportunity. When you notice you've drifted, that's a win! It's a chance to get back on track doing something you've decided is good for you.
Too, starting a new habit is an extremely personal thing! Anybody who says they have all the answers and can transform your life if only you try their system is trying to sell you something. Use new systems on a trial basis, the duration of which you should consider ahead of time. During that probationary period, get to know the system and all its parts. What doesn't work for you should be thrown out to reduce the friction of using the system. Sometimes it's by discarding something that we learn its value, and in this case you can just undelete with impunity.
- If the a system or habit stores any kind of data (e.g. note-taking, journaling, &c.), don't worry about porting everything from the old system. That will just create more start-up friction and make the new thing feel like a chore. Bring over old stuff only as you need to!