Manifesting is not about magic. It's not about winning the lottery or magicking away a genetic disease. It's about being intentional about the life you want to live and making choices and taking up opportunities that get you there.
@Ohrightyho the example you've given there about living as a healthy person is exactly it. It's not about saying it's your fault if you have a disease or illness, it's about saying if I want to live as well as I can, I am going to make choices and approach my life with an attitude that helps me to do that.
It's not about denying reality or wishful thinking. It's about taking control of your thought patterns, being aware of how we can often sabotage ourselves, and being more aware of and open to opportunities presented to us that will allow us to move towards our goals.
If the goal is to be a millionaire, relying on winning the lottery would not be a useful manifestation. That would be passive. Manifestation is about being active. What can you do to get that million pounds? Which people would you need to meet, what jobs might you need to retrain for, what opportunities might you need to say yes to, what courses might you need to take and so on - and then you get started on the road towards your goal, rather than buying a lottery ticket and hoping your numbers will get picked.
Edited to add - a lot of people never get what they want because they tell themselves they don't deserve it, they'll never get it, they're not good enough, it's impossible, etc. So they don't try. And then their lack of achievement in that area perpetuates that thought cycle.
If we want a new job but we don't believe we'll get one, and so don't apply for any, then guess what - we won't get a new job.
If we want to earn more money but we don't do anything about improving our skills, networking with the right people, diversifying our income stream or adjusting our work habits - then guess what - our income doesn't change.
Manifestation is all about telling yourself you do deserve it, you can get it, and then actively doing things to move you towards achieving what you want.