I'm always feeling unsure

Hi all. I don't know if I will be able to have anything answered here, or if what I say will make any sense but here goes. Firstly, I would like to ask the best and/or cheapest way to get a diagnosis? Followed by whatever assistance I might need should that bear fruit.

I have never felt comfortable around people, with one exception. Someone who loved me almost unconditionally, and someone who I hurt before pushing away. I have ADHD (although only started taking medication for it relatively recently,) and have had people say they think I am autistic but I have never been tested. I always joked about being on the spectrum in a satirical way that I joke about most things that I worry about but do not have enough knowledge to understand. I have been trying to give life another shot lately, after being very sad for a long time and adopting a sort-of nihilistic mindset where trying to improve my situation just did not seem possible. However, a bi-product of this 'self improvement' is that I am having to think about what it is I want from life; or what might make me happy. Firstly, the person who was in my life that I mentioned before, I cannot stop thinking about now. I have so much regret, and even when trying to study or exercise I find myself thinking about her and tearing up, it is awfully distracting. I took it all for granted during that time, she was flawless in so many ways and I didn't know a damn thing... perhaps I still don't. I would be scared to have another relationship, as I assume that someone like that is 1 in 10,000,000, and I don't know if I deserve that again, but hate the potential truth that I do not. Would it even be possible as everything seems so complicated now. Would I even be able to do better this time if someone else was special?

I have never felt like I really knew what I wanted from life. I don't really have any strong convictions about one thing or another, and my interests seems to alter from time-to-time. I always just wore masks depending on who I was with in order to fit it. I worry that whatever I do I will continue to have this fog in my head, and this sinking feeling in my heart. I would like to get tested for autism, I took a test online and scored within the Asperger's region; but I never put too much credence in multiple-choice tests, as I often find myself thinking that I could 'agree' or 'disagree' with almost every single question. I often feel like this is the case; or similar type of duality, as I feel I both hate and love myself, often leading to massive inconsistency with my thoughts and actions.

I have given myself some time to attempt to figure out what might be wrong with me, and what I have the power to improve on. Yet thinking positively still seems alien. I do not know how to cultivate that mindset, and whenever I hear someone talking about it I just think they are full of crap. It might be the case that it could work if I was able to give it a chance, but it seems at least equally likely that it is just not the method that will work for a pseudo-neurodivergent like me - at least not in the way I have tried thus far. I guess I am just always feeling unsure and confused, and the whole "that's just life" thing never sits right with me. Everything is too subjective to me. I don't know if this makes sense or it just sounds like babbling.