I slept with my best friend's ex?

I posted a question a few days ago about having feelings for my friends then boyfriend. Since then he broke up with her and admitted feelings for me. He spoke with her and told her he had feelings for someone else, but left me out of it so far.
Well today we had school together and group after so I saw him all day. We ended up going over to his house while his parents went to work. Needless to say one thing led to another and we ended up having sex for the first time. It was amazing and sorta everything I had hoped it to be. But I still don't know how to tell my best friend... and I feel like shitty person because of this
I slept with my best friend's ex?
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