Wait. What the fuck did Shepard do. All she did was touch some artifact and get visions of future reaper invasion. How did she push humanity's success. Also she didn't really foil their plan. All she did was delay the inevitable. This is one of the other reasons why I don't really like these games. Shepard is made out as some messiah of humanity when she's just one person who isn't that special besides reaper visions.
It's mostly a function of both the Council being really really badly written across all the games, and writing that relies on a string of conveniences so absurd that it's practically a comedy. Shepard receives proof of the Reapers and the Council refuse to listen to her, partially because they're all written to be idiots and partially because Shepard just says outrageous shit, offers none of the actual proof she's discovered, and demands to be taken seriously, then is shocked when they tell her to fuck off. The Council change their minds based on a scratchy audio file (lmao) but still staunchly refuse to commit any resources towards the obvious Reaper threat, beyond sending Shepard to find Saren.
Nevertheless, Shepard was, at the end of ME1, more or less singlehandedly responsible for thwarting the Reaper invasion. There were a load of ships flying around going pew-pew outside which ultimately blew Sovereign up, but everyone had inexplicably vacated the Council room where the Very Important Control Panel was. Shepard had, in a one in a billion chance, been thrown to the inside of the Citadel by the Conduit at the exact right moment for this to happen. The Normandy reached Ilos with seconds to spare, Shepard made it to the Conduit with seconds to spare (despite stopping for a long talk with a hologram), and she and her team reached the Council chambers with seconds to spare. Basically, nobody except her did anything about the Reapers prior to Sovereign's attack, and when that attack did happen, a really stupid chain of coincidences and conveniences meant that she was the person most responsible out of the entire galactic population for keeping the Reapers trapped in dark space - an accomplishment that doesn't even matter, it turns out, because they all somehow pop out of dark space on their own in ME3 anyway.
Additionally, the entire fucking fleet follow Shepard's instructions on whether or not to save the Destiny Ascension, so she's considered a hero for that too. Why does she make this choice unilaterally instead of, like, Admiral Hackett? Dunno. Maybe there's a line about it. She also gets to essentially decide on her own who the human councilor should be. It's rubbish writing, the kind that permeates a lot of fiction in general.
They see her as a messiah afterwards because of this, but also, people conveniently don't recognise her in ME2 when the plot calls for it. Stores in the Citadel delight at having her give endorsements, but thugs in t-shirts standing just round the corner don't recognise her and try to shoot at her. She's famous throughout the galaxy, but in Kasumi's loyalty mission, a gathering of socialites who are clearly on top of galactic news totally fail to recognise her when she walks in without any disguise and with her entire face visible. It's just shit, basically.