A piece of paper with instructions:

Use your tablet to take photos of your home. All rooms, and how and where you sleep. Make sure to remove the scuffed up plastic lens cover first.
At the next visit at the doctor, ask her to talk with her privately. Tell her about your living conditions, show her the photos as evidence. Don't be scared of her, she's actually a nice person. She'll try to help you.
Your mom will be mad, but most likely it's going to be worth it. Help your dad with the clean-up, I am pretty sure he's going to be the one doing that.
Oh, by the way, if you haven't yet done so, don't give your dad neigbor's WiFi password. He'll absolutely juice it by watching HD movies, and they'll get an "unexplainably" higher bill.
Also, don't get a dog. You won't have enough time for him in the future. Just get the fish.
For middle school, definitely don't listen to your dad about there being no "normal kids", go to the A grade. If your results are good enough, why not?
Lastly, here's a gift. Something you wanted so much you stole it from Tesco.

Provided with the note is something better than just the hand crank flashlight I stole. A hand crank radio + flashlight combo!

Grays Sports Almanac, 1950-2000 edition

A CD-ROM filled with advice, requirements, financial planning (nothing outrageous), a plethora of medical advice in regards to depression and ADHD, and a few books in PDF form. Also, a laundry list of "no no's".

700mb can go a long way in this situation. I've thought about it before.

earplugs

A handheld time machine

A sampler of every psychedelic I can get my hands on with a note about what shit I'm going to be diagnosed with.

Can it be my past self from a few days ago? Because I wouldn't want to give my young self any lottery numbers. That Past-Me would become a lazy ass with no values who never learns anything or knows what life is like. Now-Me would appreciate it much more.

A piece of paper with dates and lottery numbers

A rubber chicken with a pulley in the middle

A note to my 16 y/o self saying, “buy as many shares in Apple as you can.” In 1995, they were under a dollar and I could have bought thousands. Ten years later, they’d be worth enough that I could live off the dividends for the rest of my life.

A pair of boxing gloves with don't quit written on it.

my anxiety meds. my life would've been far different and less miserable if i didn't feel like death over a 5th grade fractions quiz

no-timeline-breaking mode: Buster Brown 100% cotton socks. they lack elastic and the mental relief I get from not having my ankle nerves ping my brain all day with "SLIGHT PRESSURE! HEY! HEY!!! JUST A REMINDER THAT YOU'RE WEARING SOOOOOOOCKS" is emancipating.

timeline breaking mode: detailed instructions on getting rich with crypto, majoring in computer science instead of biochem, and oh by the way you're clinically depressed, gay, and a communist just do your best to fast-track self acceptance GOOD LUCK LOL

A cheap fountain pen like a Lamy safari. Maybe some brightly coloured ink too.

Growing up I loved pens and my dad had some vintage Watermans he used all the time which were unquestionably Cool Pens but also really "fancy" so I wasn't allowed to touch them, and we just didn't know that way cheaper and less fiddly fountain pens existed because all of his came from the op shop with ink from borders and not an actual pen store. 8 year old me would've been estatic that not only do easier to use cheaper options exist, they're bright yellow and also you can put any colour in them, not just boring black.

...I feel like everyone answering "Powerball numbers" or "apple stocks" is completely missing the spirit of the question

a chef's knife

A flash drive would have saved me some trouble

My camera

A beyblade

Salbutamol inhaler.

estradiol

what small physical gift would you give your past self?

Sumatriptan

I would give my teenage self cuticle nail clippers. I wasted so much time with normal nail clippers in my youth