I feel horrible

Co

Slept with someone I’ve been sleeping with for 6 months (casual) but we’ve been inlove etc... we’re just friends now. Just found out he has been in a comitted relationship for a month. I feel absolutely horrible and would’ve never done it if I knew. He’s a pig but I don’t want to tell the girl and lose his friendship because he was my first but I know I need to and he doesn’t deserve to have me in his life.