Hi, (First of all, that's some scheisse with your ex-therapist! What a narcissistic loser)
I've managed without diagnosis, medication or specific therapy for most of my life and I've struggled but like many with a late diagnosis I was able to compensate with intelligence and wits.
It is often said that ADHD treatment has three pillars, which don't always have to come together to work: Psychotherapy, Psychoeducation, Medical Treatment.
Psychotherapy is the professional help you can get to identify your patterns of behavior that might make your situation worse and change them (and probably more).
Psychoeducation is your own learning about the condition which can be supported by therapy session but consists a lot of reading books/articles, listening to podcasts etc. Support groups can be of great help, too. They exist online and offline and you should feel comfortable with the group you find. All of these are what helped me the most.
Medication helped me, as well but only really after I properly started learning about the condition. Some people report revolutionary effects but it's moderate for me. I also had to try a few different medications and doses, which is quite common and very individual. It can also change.
There are many that share your reluctance against taking meds and ultimately, all serious therapists and support groups will support that. However, they might want to talk to you about your reasons for not wanting to try the effect at least. A reason is, that they can often be a jump start for successful therapy because they can reduce the primary symptoms of ADHD (like inattentiveness) and let you work better on the secondary symptoms (e.g. low self-esteem), both of which differ from person to person. Even if you decide going without meds, it's a good step to know what's your concept of yourself and your mind and how meds would interact with that because it gives you a better understanding of your personal situation.
I hope that helped a bit even though I didn't go into how I managed myself before the diagnosis in my late 20s. It's been rough, to say the least. It still is but I have learned a lot about myself that lets me accept myself more easily for who I am (a sentence I would have laughed about in the years before). And that in turn makes everything a bit easier.