I think it's great that you recognise that worries and doubts are normal and signs that you are indeed considering everything carefully
I didn't know for sure that I would be able to parent well and avoid making massive mistakes (as opposed to the bad days everyone will have). But I knew that I wanted to be a mum, more than anything. And I knew I would committ to any child that moved in with me and hang on with everything I had if things didn't go so well. I didn't know how much I would be able to handle, but I knew I would do my absolute best. I had worked (a bit, not a great deal) with some children in care and so I knew a little bit about what issues they had, and so I was a bit nervous about living with those issues full time, but actually meeting the children and so knowing them as, well, them, and not just on paper with a long bullet point list of their difficulties, also gave me confidence that there would be good times and experiences with parenting - I could see their strength and great qualities as well as their difficulties, I could imagine what it would be like to parent these children - and the more time I spent with children, the less I could imagine not having a child of my own. By the time I was doing the homestudy visits, I wanted to be a mum so much that I couldn't imagine changing my mind and not going through the rest of the adoption process and staying childless
I was naive about what parenting would be like, yes, and that wasn't a good thing
But on the other hand, however much you know, you can't actually truly know what it will be like until you do it
And I don't regret my decision for a second - I'm a mum. That's just the most amazing thing in my life 
Whilst most adoptive parents are relatively resilient people, there are extremely few who actually possess super human levels of resilience and patience etc. Nearly everyone has normal levels of resilience and patience and empathy. There are certain qualities you need to have, but not in such amazingly huge quantities that you are unlike everyone else around you.
Also, I would say that whilst in my experience, the majority of children have some additional needs, it's a minority of children who have very significant/severe needs. Although you can't know what difficulties your future child will have, thinking that all children have needs that are pushing their parents to the brink, is not realistic. The large majority of adoptive families I know, here and online, do not regret deciding to adopt, have children they love with eveery fibre of their being, and do have a family life, even if it's not the same as other families.