A diagnosis can help because it would mean you could access medication, which is basically the most effective treatment.
Another option I've learned about recently is Neurofeedback. As it's not covered by the NHS anyway, I don't know whether you'd need a diagnosis. It's not cheap though but it's supposed to be about as effective as medication. On the ADHD forums response is mixed - some people are sceptical because it tends to wear off. But some people report that the treatment can last for years, which is obviously more long-lasting than medication which will stop working the day you stop taking it. If you do look into this, beware of predatory clinics jumping on this bandwagon as it is NOT regulated. You must be able to sit down and have a proper discussion with somebody to work out whether it would be useful for you and get a real sense that they only want to help people they can actually improve things for, not just sell it to you regardless.
Here is a guide to getting diagnosed in the UK: aadduk.org/faq/
I haven't been able to try medication even though I've been diagnosed just over 2 years now. First I kept procrastinating/forgetting to make appointments (classic, helpful) and then I got pregnant. I miscarried but was stupidly honest about TTC and they wouldn't touch me. Now pregnant again so I'll try again when this baby is born and hopefully find somebody more local. Unfortunately in Germany where I live the only allowed medication is methylphenidate (ritalin) whereas in the US apparently they tend to start you off on vyvanyse now, which I forget the chemical name. Or adderall which might be the same thing? I'm not sure.
Things which have helped me without being medicated:
Dr. Russell Barkley's lectures (on Youtube). They are stupidly long so you need to find a way you can focus on them. For me the only way I could process them was to listen in chunks while I was doing something else with my hands/eyes but not brain, like ironing or folding clothes or tidying the house. Very emotional actually because he understands ADHD like nobody else I've come across. I have got his book too but it's nowhere near the level of the lectures, IMO.
The reddit subforum r/ADHD. Lots of understanding and support here even though the majority of posters seem to be American men aged 16-25. There is a smaller women's one called r/TwoXADHD but it isn't as active. The main sub has the better content, though the women's members will always reply, if you have a question to ask them.
Any reading about executive function. This is Barkley's key to his ADHD model. Understanding exec func deficits has helped me to accept what I can control, what I can't, what I should be able to rely on in my brain, what I likely can't - even though others can. That has helped me to put systems in place and adjust my expectations.
The youtube channel "How to ADHD". I found Jessica to be annoyingly upbeat at first but give her a chance, pick a video on a topic which seems useful and watch it all the way through. I actually appreciate that she makes her tips really bite sized and keeps the videos interesting. The peppiness has stopped irritating me now, because the content is just fantastic. Practical, relatable, yes.
Less helpful/to look into:
I did CBT but I found it frustrating and unhelpful. I don't think my therapist was very good. He ordered me a book which I think would have been very useful IF I had been medicated too, because it kept going on about the importance of combining CBT with medication and this dual pronged approach. The idea being that unless you were medicated in childhood, you've developed awful habits/coping mechanisms and the medication doesn't magic those away. So you use the CBT to develop good habits, but these won't stick without medication. So you need both.
This article which came up on r/ADHD the other day but I haven't got around to reading yet: www.guidelinesinpractice.co.uk/neurology-/practical-implementation-tips-adhd-in-adults/453652.article
(Actually, I thought that was about management but it's more medical but it is UK based! So I'll leave it in.)