I remember Fabio Capello telling the story of how when he went to manage Real for the first time in 1996 everybody at the club (even the driver) was saying to him "Welcome to the best club in the world" This is the level of mentality we are talking about there.
I absolutely agree that Barcelona's confidence depends of how good they play their own style. This is the Cruyff legacy cemented by Guardiola there so at least for the forseeable future that will continue to be the standart they need to follow and maintain if they want to remain a top club.
I disagree with this.
When it comes to Europe/CL and general KO rounds one of the biggest deciding factors is the individual quality of the players, RM in the history of football has boosted some of the best in the world and they manage to get them surrounded with some of the best supporting cast. di Stefano has Puskas, Gento, etc they went on to win 5 European Cups, Zidane had Roberto Carlos, Figo, Raul, etc won 2 CL and CR7 with Kroos, Modric, Ramos, Marcelo, etc. won 4 CL titles. Even Barcelona from 2006-2015 They won 4 CL titles and 2 historical trebles, while they've boosted Messi, Dinho, Etoo, Henry, Xavi, Iniesta, Toure, Puyol, Villa etc. Milan have many European Cups/CL with Maldini, Van Basteen, Rijkaard, Kaka, Pirlo, Nesta, Cafu, Shevchenko, etc all have around 5 European Cups/CL between them.
If you look at United and Juve as an example legging behind them, they've never really boosted this level of individual quality. In KO rounds lacking individual quality is more punishing as there will come time you need that brilliance from your player, no tactic, philosophy etc will bail you out.
Rather you go to United, Barcelona, Juve, Bayern Munich etc they all know/think they're best in the world and boost that mentality.