Can’t speak to ADHD in particular and don’t want to discount the difficulty it adds, but one of the biggest hills I always have to get over is not judging my own thoughts. It’s easy to get frustrated that you aren’t able to achieve a more still mindset but at the end of the day the thoughts of judgement are just thoughts that can be observed like any other.

Try it like this: go for a walk in nature and focus on the senses. Try to really feel yourself walking, feel the clothes on your body, the wind, the distant sounds. As things enter your perception, be grateful for them. The dogs and children, the leaves and sun. Perhaps contemplate how: (1) this sensation (of ex "dog") feels inside you (2) the miracle of its construction, the billions of particles, the quantum effects underneath, and the orchestral perfection of its movement (3) all the relationships people have with their dogs (4) how dogs affect the entire world

This technique gives you a large "surface area" for gratitude droplets to coalesce. With a little repetition, you can get very high doing this technique, making it fun and self reinforcing. Ultimately all sensations can be integrated into your larger self-experience, changing the small, separate identity, into a large all encompassing compassionate one

My understanding: the fundamental skills are attention (focus/zoning in), awareness (broad attention of sensation), and gratitude (compassion/metta/love). You can train these skills whenever is the most enjoyable, including painting, dance, gardening, working out. Gratitude is the one that makes the most happiness up front, check out the "hedonic treadmill"

Have you ever tried Wim Hof breathing techniques? I also have ADHD and I have a really hard time with meditation but it works for me.

It also sounds like it could be possible that this is at least partially an issue with your body chemistry. There are so so many little things that can affect the brain, like maybe a medication or any other drugs you might take, a food you're not fully aware that you're a bit allergic to, inflammation, a malfunctioning brain chemistry such as an anxiety disorder, etc.

Here's a link to the Wim Hof video if you're interested. Just be aware that people get weirdly cultish about him, and I'm not sure I would follow everything he recommends, but this particular exercise helps my anxiety a lot.

https://youtu.be/tybOi4hjZFQ?si=yHTjE7QlNITNP6ff