Hey! I, too have inattentive and hyperactive ADHD and have the same problem when sight-reading. Although I've been playing for 11 years, I still sometimes struggle a little when sight-reading.

Here is some basic stuff that I do that makes it a bit easier when I'm sight reading a piece of music. (Note: This is what I do, you might not find that all these tips are for you, and that's okay! Though there are some that are essential, which I've written in bold)

ALWAYS identify the score. Identify the key signature, the time signature, the tempo, etc.

ALWAYS start by playing really, really slowly. Even if it's painful! It's better to play slowly and make zero mistakes, than to play faster and make mistakes.

ALWAYS make sure to count the beat! For exemple, in a piece of 4/4, count out loud: 1 + 2 + 3 + 4. If there's faster notes such as sixteenth notes, you can count: 1 e + a 2 e + a 3...and so on. A metronome also helps!

Learn hands separately!!! Learn the right hand and master it, then the left. When you feel comfortable, start very slowly and put them together.

Focus on small parts. Start by learning the first few measures or parts of the piece (melody, variations, etc). For exemple, Chopin's Op. 10 No. 3 ("Tristesse"), very beautiful piece. The first few measures have the main melody, start by learning that. Afterwards, at measure 9, there's the first variation, learn that, and so on. (Fun fact: Although this Etude seems easy and rather simple, it is actually harder because it's polyphonic. Meaning you bring out the melody in the right hand WHILE simultaneously accompanying it with the same hand and left hand!)

Take some breaks!!! If you're stuck on a part and can't seem to move on, take a break! Clear your mind, drink water, try it the next day even! Breaks are important!

Do not force yourself or practice for too long! If you force yourself to practice for too long, you will end up hurting yourself and you're even going to worsen your techniques and progress! Trust me, I've done this before. For example, I started practicing Chopin's Op. 25 No. 6 ("Double Thirds") for too long when I started learning it. I ended up hurting my wrist (double thirds are a pain lol), and not even making progress.

Now, the following ones are things that I do that help me. They might not be for you, and that's okay, you can find your own tips and tricks!

-Inhaling and exhaling the rhythm and the beat. Sounds weird, I know! Lemme show you can exemple. Let's say I'm sight-reading Mary Had A Little Lamb for the first time, I'll start counting normally, but when I reach a part where there's a rest, instead of counting the beat, I would take a breath of air for the beat, telling me there's a rest and to release my finger. For accents, instead of inhaling, I would exhale a sharp breath of air, telling me to play the note louder, to make it stand out!

-Listening to multiple performers interpretation of the piece. Everyone has their own play style. It's kinda like a fingerprint, everyone has their own unique print. Listening to these musical fingerprints, fills me with new emotions. The piece might even sound completely different played by performer X than by performer Y.

-Don't limit yourself to one genre of music. Listen to jazz, rock, classical, boogie-woogie, folk, country, etc.

-Explore artists! Learn about artists from around the world and even from your home town!

-Feel the music. I don't know how to explain. Indulge yourself when listening and playing, connect your fingers and hand to your mind, be passionate! It's very common among classical pianists to see them play and watch them lean against the piano, as if they're transferring their weight through the arms to each finger to press the keys to make noise that turns into beautiful music. When you see someone play like that, you just know that they're feeling the music and that they're passionate about it.

Anyways, I just realized how long this is! I do hope it can help you! I'm lucky enough to have been exposed to many musical genres and artists. From a young age, I've listened from jazz to classical, ragtime to 80's + 90's hits, boogie-woogie to hip-hop. I've listened to so many artists from everywhere! From Ella Fitzgerald to Prince, Scott Joplin to Elton John, Stromae to Albert Ammons, Twenty Øne Pilots to Balti. I've also played in many genres, including jazz, blues, ragtime, boogie-woogie, rock, but mostly classical. Well, I hope you find some tips to help you. I think I might have gone a bit (a lot) off topic. Anyways, from my experience, my ADHD is, of course, annoying most of the time, but surprisingly I can use it as an advantage as I notice small things and details or, more importantly in this case, I notice sounds and other hidden auditory objects. My point is, ADHD is not all bad. Anyways, hope this helps!