Yep. I made the same mistake my favourite remixer [GlitchxCity, she did the remix above ♥] made, miraculously, and it made me feel significantly less like a murderous asshole for accidentally killing Toriel. SERIOUSLY IT SOURED THE MOOD FOR LIKE THE ENTIRE RUN. Every inspiring thing someone said made me think "yeah but I still killed Toriel tho"
LISTEN I got extremely attached. She reminds me a lot of a character I've written.
Anyway I started a second run immediately, saving after I didn't kill Toriel. Not fucking up this time. Going to attempt an all-mercy run on my own the first time, and if I fuck it up, I'll consult a guide in the next attempt.