So you mean demo account is not on forex at all and has different laws? Or you simply mean that price movements are never 100% sure and holy grails dont exist?
You will not find a system that has a 100% perfect always win history.
Holy grails do not exist.
I suppose that in theory they 'could' exist, but the likely hood of it being ever made available to the general public would be infinitesimally small. Why would anyone release it and make it available? They'd basically have infinite wealth at their disposal.
which is not solid enough imo to prove that demo is totally different.
Demo account price feeds are often different from Live account price feeds.
Different spreads, different response times, etc etc etc.
If I develop my own personal working strategy on demo account I am 100% sure that it is going to fail on live account because demo is totally different? Is there any guarantee in forex at all, at any time?
If you feel 100% that you're going to fail on a live account, then you should absolutely NOT put money in your live account.
You asked if there is any guarantee at all? NONE. There is no guarantee of making money with forex.
If you think you are going to fail, then you've already failed, and should probably keep learning and practicing your strategy, or not trade at all.
If it looks too good to be true, it's probably a scam! Let the buyer beware.