I can't give you much advice but thought it might make you feel better if I said I've done the same thing.
An ex kept mentioning (while we were together, not that sad haha) that he liked blond hair (needless to say, that relationship is long since over) so I did similar with a pre lightener.
Had to run out before the shops shut in a hat and buy a dark colour to go all over and had, rather fortuitously, booked in for a trim the next day. It took them 3 hours to fix, they had to get all the dye out then put another colour on top, it wasn't pretty...or cheap...never tried that one again :)