You and Shigaraki were close friends and he trained you to fight and taught you everything you know. Eventually you two fell in love and started dating.

Then when All for One had Dr. Garaki did his ‘upgrade’ on Shigaraki to make him stronger he also made him forget all his memories with you per All For One’s request. AFO had him do this because in his eyes nothing should come before Shigaraki’s mission and especially not something as ‘weak’ as love.

Because of this you defected to the hero’s side and during the war you ended up having to fight Shigaraki but were losing quite badly since he taught you everything you know he knows exactly how to counter all your attacks.