Board Thread:Questions and Answers/@comment-29430569-20160803190220/@comment-29430569-20160816174702

Mabian wrote: You likely got what is commonly referred to as the Pacifist Neutral ending. The Neutral route is completed upon defeating Flowey (regardless of whether you spare or kill both Asgore and Flowey).

You should be able to reload your save before the Asgore fight and complete the True Pacifist ending without needing to restart the whole game. Just make sure that you have befriended and hung out with both Papyrus and Undyne, and from there you'll be able to complete the last few pieces. Thank you! It realised later that I hadn't befriended Undyne anyway so, oops..

Oh and also it wouldn't let me reload the save before the Asgore fight :/