Its this https://www.fanfiction.net/s/8856198/22/Reversal-Time
Its in rusian, but the chrome translator did a good job of translating to english.
Its a fanfic about 14 year old shinji, before going past the point of no return with sakura after learning the truth, summoning his post UBW-self (who became a Counter Guardian) with a botched ritual, and accidentally had him posses his body. Whith him acting as a mentor figure and soul resonance shenanigans.
I want to know what people think of the ending.