One further point, and I think for writers who aren't terribly confident, it's an important one.
Nobody can give you anything more than advice: there are no cast-iron guarantees of success (however you define the word). All you can do, as a writer, is to try to maximise your chances by producing a high-quality, readable book about something that people will enjoy, that entertains them and doesn't drive them away.
The internet is full of people whose standard response to "Is this any good?" is "No! That is wrong! Books do not do that!". I tend to think that a response in these terms, even if it's correct, is basically worthless. Nobody can really have that level of certainly. The closest you can get is "This doesn't work for me, and I'm pretty sure it won't work for anyone else". It is probably the case that unless what you've written is mind-insultingly wrong, in that no sane person could think like that, then it's time to rewrite it, or to consider a different angle, rather than to give up entirely.