But where to even start?

If you live somewhere with socialized healthcare, start by visiting your general practioner, your regular doctor or "doctor's house" and say you suspect you relate to having executive dysfunction (ED). Mind you, ED can be symptom of other things than just ADHD (like ASD), although that is the most common culprit. Hopefully they'll advice you on where to go next, in my case it was a psychiatrist where I had a waiting list of almost 2 years, but if you go private it's probably a lot faster.

(US only) Contact insurance, ask for a psychiatric nurse practitioner or med manager. They have the degrees to prescribe meds, which are a LIFE CHANGER. the first time I took a stimulant I ended up working on paperwork for like three hours straight, I could actually focus. I didn’t even realize my background YouTube vid ended for like an hour. Once I realized that I was able to focus for that long so effortlessly I genuinely cried.

Also, a 1-2 punch of meds and therapy is best.